National Park Service via AP file

A deactivated nuclear launch facility near Wall, S.D., similar to classified facilities at Minot (N.D.) Air Force Base. All 17 of the AirForce officers disciplined last month are assigned to Minot.

A "breakdown in overall discipline" led the Air Force to suspend 17 officers and disqualify them from controlling nuclear missiles after a poor inspection at one of the service's most important nuclear bases, military officials told NBC News on Wednesday.

All 17 officers are assigned to 91st Missile Wing at Minot Air Force Base in North Dakota, headquarters of the 5th Bomb Wing, which maintains 150 nuclear-tipped Minuteman intercontinental ballistic missiles.

The unit received a "D" rating during inspections in March, leading senior officials at the base to call for an immediate crackdown, NBC station KMOT of Minot reported.

The inspection was only the latest in a series of high-profile failings, which led the group's deputy commander, Lt. Col. Jay Folds, to send an internal email complaining that the unit is suffering "rot" within its ranks, according to a copy obtained by The Associated Press.

"We are, in fact, in a crisis right now," Folds wrote, according to the AP.

The 17 officers — representing almost 5 percent of the 91st Missile Wing's missile launch staff — were suspended for 60 days last month and were stripped of their authority to control and launch nuclear missiles after the "D," or "marginal," inspection in March, an inspection the Air Force publicly labeled a success.

Air Force and other Defense Department officials confirmed the basic details of the AP report, telling NBC News that a "breakdown in overall discipline" led to a series of relatively minor violations of rules and procedures.

The base's nuclear weapons and facilities were never in jeopardy, they said. But the combined potential impact of the violations raised serious concern within the base's leadership, they confirmed.

The base has come under scrutiny for other incidents in recent years that raised questions about its security and oversight.

In August 2007, six cruise missiles loaded with W80-1 nuclear warheads were flown from Minot to Barksdale Air Force Base in Louisiana before authorities discovered that the warheads hadn't been removed for safety, as required.

In a review of the incident in February 2008 (.pdf), the Defense Advisory Board blamed "process and systemic problems" that had "developed over more than a decade and have the potential for much more serious consequences."

But just five months after the blistering report was issued, in July 2008, three Air Force officers fell asleep at the controls of a component that contained old launch codes for nuclear ICBMs. They were immediately barred from working with classified and nuclear materials and were later discharged from the service.

The new report comes as the base is breaking in a new commander, Col. Alexis Mezynski, who took over in January and must now work to put out the fire.

In an interview with KMOT as he assumed command in January, Mezynski acknowledged that there were problems at the base, which was built in the 1950s and needs to be brought back up "to conditions that are workable."

"There are going to be changes," he promised at the time.

Jim Miklaszewski and Courtney Kube of NBC News contributed to this report. Follow M. Alex Johnson on Twitter and Facebook.

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Brendan Mcdermid / Reuters file

An Amtrak police officer watches as passengers prepare to board a train at New York's Penn Station on April 19.

WASHINGTON - An alleged al Qaeda-backed plot to derail a U.S. passenger train in Canada sought to exploit the vulnerabilities of railroads that have not gotten much attention from the American public. 

While the United States has sharply tightened security around airlines since the September 11, 2001, attacks, trains are far harder to police, with masses of passengers getting on and off and stops at many stations on a single line. Thousands of miles of track, bridges and tunnels present a major challenge to monitor.

Even though the United States has largely been immune from attacks, extremists around the world have frequently exploited rail transport's vulnerability, said Brian Michael Jenkins, a security expert with the Mineta Transportation Institute at California's San Jose State University.

"Surface transportation really has become the terrorists' killing fields," he said.

Two suspects were arrested in Canada on Monday charged with conspiring to blow up a trestle on the Canadian side of the border as the Maple Leaf, the daily Amtrak connection between Toronto and New York, passed over it. Amtrak is the U.S. passenger rail service.

The two men charged in the plot made their first court appearances on Tuesday. A lawyer for one said his client would fight the charges vigorously.

Jenkins and Steve Kulm, an Amtrak spokesman, said trains presented a unique security challenge, different from airports with their screening process for passengers.

Amtrak coordinates security with local law enforcement, does counterterrorism exercises and patrols its tracks and stations, Kulm said. It also is reconfiguring stations to make them safer from potential attack.

"It's no surprise and no secret that overseas terrorists have targeted rail transportation, and so we have, as I say, many seen and unseen measures that we have put in place and continue to improve upon," Kulm said.

More fatalities in surface attacks
Although popular attention has tended to focus on airliner attacks, far more people have died worldwide from surface transport assaults, Jenkins said.

Since the Sept. 11, 2001, militant attacks on the United States, there have been 75 assaults on airliners, with 157 fatalities, he said.

During the same period, there were 1,800 attacks on surface transport, with nearly 4,000 people killed. Among them were attacks on Madrid in 2004 and on Mumbai in 2006 that each killed about 200 people, and a 2005 London bombing that claimed 52 lives.

In the United States, only one person has died from an extremist rail attack in recent decades, when Amtrak's Sunset Limited was derailed in Arizona in 1995. Responsibility was claimed by a group calling itself Sons of the Gestapo and the saboteurs have not been found.

The United States has more than 200,000 miles of railroad, with about 21,000 miles used by Amtrak. Amtrak carried 31.2 million passengers in the last fiscal year, its ninth record year in the last 10, Kulm said. As a comparison, about 642 million passengers were carried within the U.S. by airlines in 2012, according to the Department of Transportation. 

Elliot G. Sander, a former chief executive of the Metropolitan Transportation Authority of New York, which runs two of the biggest U.S. commuter railroads, said public awareness was critical to countering potential attacks.

"One cannot understate the importance of the participation of the public, in terms of eyes and ears," he said.

Far fewer security personnel
The Department of Homeland Security spent $136 million in the 2013 fiscal year on surface transportation security, with 775 personnel. Aviation security received $5.3 billion and has 53,000 personnel.

Special Visible Intermodal Prevention and Response teams carry out random baggage and security checks at train, subway and bus stations as well as at truck weighing stations.

Stephane Jourdain / AFP - Getty Images file

An Amtrak police officer and a sniffer dog patrol at Union Station in Washington on May 6, 2011, five days after Osama bin Laden was killed in Pakistan. Intelligence seized from his compound showed al Qaeda pondered strikes on U.S. trains on the 10th anniversary of the Sept. 11 attacks, U.S. officials said.

Created after the Madrid railway bombing, the VIPR teams carried out more than 9,300 operations in fiscal 2011, according to the Department of Homeland Security's 2013 budget request.

The Transportation Security Administration was criticized last year by the Government Accountability Office, an investigative arm of Congress, for failing to carry out analysis of railroad security information.

The GAO also criticized the TSA for inconsistent reporting requirements from rail agencies and failure to inspect a rail service the GAO did not name. The TSA concurred with the GAO's recommendations for improvement.

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The arrest of Dzhokhar Tsarnaev ended the manhunt for the Boston Marathon bombers, but it set in motion an equally intense phase of the case that will begin with the grilling of the man who – for now at least – is the only surviving suspect.

An indication of the complex investigation ahead came Friday night, when an Obama administration official told NBC News that Tsarnaev would not be given a Miranda warning when he is physically able to be interrogated after receiving medical treatment.

Instead, the official said, the government will invoke a legal rule known as the "public safety exception," which will enable investigators to question Tsarnaev without first advising him of his right to remain silent and to be afforded legal counsel.

The exemption can be invoked when information is needed to protect public safety. In this instance, the government believes it's vital to find out if Tsarnaev planted any other explosives before his capture or whether others might have plotted with him to do so, said the official, speaking on condition of anonymity.

Tsarnaev’s older brother, Tamerlan, was killed in a shootout with police early Friday, and it was not clear until late Friday that authorities would be able to question their remaining prime suspect.

Until shortly before his capture around 8:45 p.m. ET, the wounded and bleeding Tsarnaev exchanged gunfire with authorities in Watertown, Mass., while sheltering in a plastic-wrapped pleasure boat.

Officers on the scene and the brass in the command center were both clearly elated by the outcome.

“We always want to take someone alive so we can find out what happened,” Boston Police Commissioner Ed Davis said at a media briefing an hour later, “and we can hold them to justice."

High Value Detainee Interrogation Group
The rule waiving the Miranda warning does not set a precise limit on how long a suspect can be interrogated before being advised of his rights, but it likely buys authorities no more than 48 hours.

During that time Tsarnaev, 19, will be questioned by a federal government team called the High Value Detainee Interrogation Group, consisting of officials of the FBI, CIA and Defense Department. Though he will not have a lawyer present, any statements he makes during the questioning will be admissible in court.

Among the questions investigators are certain to focus on is whether he and his brother  had help in plotting or carrying out the terrorist attack at the finish line of the marathon. The dual blasts from pressure cookers packed with explosives and shrapnel killed three people and injured 176.

That question took on more urgency when police in New Bedford, Mass., south of Boston, announced Friday evening that three people there had been taken into custody as part of the bombing investigation.

In addition to possible co-conspirators in the U.S., the interrogators also will want to know whether the brothers, both ethnic Chechens, received any assistance from overseas.

Travel records obtained by NBC New York showed that Tamerlan Tsarnaev left the country for six months in 2012, flying to Moscow on Jan. 12 and returning on July 17. Where he went and what he did after his arrival in Russia could expand what so far has been a domestic manhunt into a global one.

Enemy combatant?
Suspicions that the elder brother could have received terrorist training or support abroad were heightened Friday, when an official familiar with the matter told NBC News that a foreign government had expressed concern in 2011 that Tamerlan Tsarnaev could have ties to terrorism. The official said the FBI investigated, but found no such links and reported the findings back to the foreign government.

Even if authorities determine that the Tsarnaevs received support from an overseas terrorist organization, the Obama administration official said the government will not seek to declare him an enemy combatant and try him before a military commission, as it has done with senior al Qaeda officials captured overseas and imprisoned at the Guantanamo Bay detention camp. Administration officials see that scenario as a non-starter, the official said, particularly given the fact that Dzhokhar Tsarnaev is an American citizen, naturalized last September.

AP

Tamerlan Tsarnaev, left, was killed by police. Dzhokhar Tsarnaev was captured and will be interrogated by a special team of investigators.

Republican Sens. Lindsey Graham of South Carolina and John McCain of Arizona issued a statement late Friday urging that the administration hold Tsarnaev as an enemy combatant.

"It is absolutely vital the suspect be questioned for intelligence gathering purposes. We need to know about any possible future attacks which could take additional American lives," said the statement, posted on Graham's Facebook. "The least of our worries is a criminal trial which will likely be held years from now." 

Mass of evidence
At the same time they are seeking to uncover the bombing suspects’ motives and determine whether they had a support network, investigators will continue to collect and analyze vast amounts of forensic evidence from crime scenes stretching across three cities.

In addition to processing evidence from the bombings, FBI technicians will analyze hundreds of hours of video camera recordings from private and public surveillance and traffic cameras as they attempt to trace the brothers’ movements – both after the attack and before it.

Investigators also will obtain and assess phone records, seeing who the brothers were in contact with in the weeks and months leading up to the attacks.

Only when they have scrutinized every bit of data, and explored every lead, will they turn over the mountain of evidence they have assembled to prosecutors. It will be up to them to decide what charges the younger Tsarnaev should face and whether to seek the federal death penalty in a state where life in prison is the maximum sentence that can be imposed.

But despite such a massive expenditure of time and technological know-how, they may never answer the most haunting question surrounding the case, as President Barack Obama noted.

“Why,” he asked during a brief statement on Dzhokhar Tsarnaev’s arrest late Friday, “did young men who grew up and studied here as part of our communities and country resort to such violence?”

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If Boston fans are allowed into TD Garden on Wednesday for a scheduled Bruins-Sabres game, or into Fenway Park on Friday for the next Red Sox home game, they are sure to be greeted by grim reminders of Monday's terror attacks — beefed up security, extra uniformed police, additional bomb sniffing dogs. Already, fans around the country are seeing and feeling the aftermath at sports venues around the country.

Experts are wondering if these changes are temporary or permanent. Could the Boston Marathon bombing to do sporting events what the 9/11 attacks did to airports? Might the attack lead to aggressive new security measures that create long security lines and frustration for fans?

One change could come quickly: Sports venues around the country might take a hard look at banning all backpacks or bags of any kind, said Dr. Lou Marciani, director of the National Center for Spectator Sports Safety and Security at the University of Southern Mississippi, which is funded by the Department of Homeland Security. The center trains venue operators and security personnel around the country.

At least one of the bombs that detonated near the finish line of the Boston Marathon on Monday was hidden in a backpack. Investigators have found fragments of dark or black nylon, which were possibly from a bag or backpack that contained the bombs. 

Whether or not backpacks were used, bags are sure to come under increased scrutiny.

"Some stadiums are moving toward eliminating any type of carry-in anyway," said Marciani. "If that ends up being a factor (in this attack), that might be one best practice that comes out of this."

Those attending sports events for the next several weeks should expect to see heavy security presence, Marciani said. Fans should leave a little early and pack some extra patience, particularly in Boston and other East Coast cities.

Marciani urged careful deliberation, however, as little is still known about the methods of Monday's attacker.

"We can't do anything until the investigation is complete," he said. "When it is, we will take a look at that information, we will collaborate at our annual conference, and digest that into our best practices."

'It's not fool proof'
One security measure that is unlikely to appear at sports venues: metal detectors. Bob Karl, who runs security firm Safety Act Consultants, said the machines cost $35,000 each and would be too costly to add at every venue gate. 

"And it's not fool proof," Karl said. A magnetometer would catch a bomb packed with metal shrapnel, like the ones apparently used in the Boston attack. But it's not hard to craft bombs that are invisible to metal detectors. The next step would be X-ray machines, which no one wants at sporting events, he said.

"You can really create some unhappy fans ... How many people per minute can you screen, and at what point is the fan experience ruined?" he said. "You are always balancing the threat versus the inconvenience and the expense."

Karl agrees a backpack ban will get fresh consideration, and thinks some other lower-cost technologies might be implemented at future events.

"Supposedly, this device was in a trash can," he said. "At a lot of the high-rise venues I work with, any trash cans there are blast resistant."

Urban sports venues like Yankee Stadium pose their own security challenges because they don't have a natural buffer zone around the perimeter that can be secured — such as the parking lots which surround many suburban stadiums. Additional security personnel — and diligent fans — are the best security measures, he said.

“We'll look at some training, but we're not going change what we're doing now," he said. "We will keep our public events in this country. One thing I can take away from this is we are proud of what the response was. We've trained a lot of first responders, and they were ready."

Slideshow: Boston Marathon explosions

Charles Krupa / AP

See images from the scene of the explosions.

Launch slideshow

'Do the best you can'
There are special problems with securing a marathon, Marciani said, and it would be a mistake to draw too many conclusions from Monday’s attacks. Marathons have more in common with public space events like parades than big-ticket sporting events like pro sports, he said.

"Marathons are open-ended. There's no access control. You have 26 miles to cover. It's a lot more like St. Patrick's Day," he said. At stadiums, every entrant must pass some through some kind of access point. "(At a marathon) you do the best you can, check the manholes, check the garbage cans, but you can't manage every aspect of 26 miles."

David McWhorter is a sports stadium security expert with Catalyst Partners in Washington, D.C., a consulting firm that just helped Yankee Stadium operators obtain the coveted "Safety Act" designation, meaning it has been deemed well-protected against terrorist attacks by the Department of Homeland Security.

There are additional high-tech measures that marathon operators — such as those running this weekend's London race, or Montreal's race on April 28 — might consider, he said.

"A huge cadre of K9s might have picked up on something. Also, depending on the viewing angle and other considerations, there is software ... that analyses camera feeds and can detect certain types of movement or unattended items," he said. "Also, there are more covert technologies that monitor for certain electronic signals, but I'm not at liberty to get into details.

"Lastly, there is the option of more human security guards, although it is not at all clear that this would have helped," he said. "Unfortunately, all those things take money to buy and maintain. No one has the budget to defend against all possible threats."

Follow Bob Sullivan on Facebook or Twitter.

Had the Boston explosions happened a few years ago, the buzz would soon be about the color orange. As in “Code Orange” — the government’s way of indicating a high risk of terrorist attacks under the oft-maligned color-coded Department of Homeland Security terror alert system.

Today, the color chart is history, replaced in 2011 by the National Terrorism Advisory System.

So what should travelers know about the state of airport and mass transit security in the wake of the blasts?

No national alerts have been issued, according to the DHS website. In fact, the agency has never issued an alert under the system, which requires the government to send out “formal, detailed alerts” when it receives information about a specific or credible terrorist threat.

The agency did not respond to an on-the-record request for comment about the process, but in a statement issued Tuesday afternoon, Secretary of Homeland Security Janet Napolitano said there is nothing to suggest at this point that the events in Boston are indicative of a broader plot.

“Out of an abundance of caution, DHS continues to keep in place enhanced security measures at transportation hubs, utilizing measures both seen and unseen,” Napolitano said.

She urged the public to remain vigilant and immediately report any signs of suspicious activity. 

“It is notable that they have not used the (NTAS) system to date,” said Christian Beckner, deputy director of the George Washington University Homeland Security Policy Institute.

“The original color coded Homeland Security Advisory System was so broad that it didn’t lead to effective reactions. This more targeted approach is the right one.”

After the deadly Boston blasts, a number of airports are either beefing up police presence or operating under a “heightened state of alert,” including the three major New York-area airports and Los Angeles International. Airlines are extra vigilant, too, and not taking any chances with suspicious items. On Tuesday, a US Airways flight was remotely parked at Boston Logan International “out of an abundance of caution” so that a bag on board could be examined. The baggage was deemed harmless and the flight taxied to the gate. Airport operations were not impacted.

Related: Reports of suspicious activity spur travel headaches

Regardless of whether or not the government issues a national terror alert, TSA can independently send a security notice to the airlines and recommend additional security measures at airports, Beckner told NBC News.

“That’s more precautionary, not necessarily tied to any specific thing,” he said.

The TSA declined to comment about how the Boston explosions are affecting airport checkpoints, referring all questions to the Department of Homeland Security.

If the government issues an alert under NTAS, it will specify whether there is an “imminent threat” or “elevated threat.” The alert will also explain the potential danger, outline what actions are being taken to keep the public safe, and recommend steps that you can take to protect yourself.

“It’s a vast improvement over the last system,” said Rick Nelson, a senior associate of the Homeland Security and Counterterrorism Program at the Center for Strategic and International Studies.

“I don’t think anybody misses the color coded system except for the late night comedians.”

Each threat alert will have an expiration date so the public doesn’t have to deal with an elevated threat level for an unending period of time, Nelson said. That’s quite a contrast from the previous approach, implemented soon after the 9/11 attacks. Road warriors may remember that the threat level in airports remained at orange for years under the color-coded system.

“It wasn’t really based any credible threat and it really didn’t tell people what to do,” Nelson said. “If I told you the threat level was now red, what did that mean to you as a citizen and what should you do differently? That was always very unclear.”