Charles Krupa / AP

A woman carries a girl from their home as a SWAT team searching for a suspect in the Boston Marathon bombings enters the building in Watertown, Mass., Friday, That was part of what turned out to be a chaotic week in the U.S.

By Bill Briggs and JoNel Aleccia, NBC News

Americans found their resilience pushed to the limit  this week – and they still don’t know what’s coming next.

When the Boston Police Department tweeted "CAPTURED!!!" Friday night, signaling the apprehension of the second suspect in the bombing blasts that devastated that city's famous marathon, their elation was echoed by people across the nation who clapped, cheered, pinged, Facebooked and tweeted their own relief that, finally, there was an end to the manhunt -- and a hellish span of days.

Even though that siege has passed, the impact of collective crisis fatigue may well linger, experts say.

The U.S. already had endured Monday’s deadly attack, Tuesday’s poison letters and the Wednesday Texas fertilizer plant explosion that has left a still-untold number of people dead, 60 missing and 200 injured. Thursday and Friday saw a late-night shootout and a day-long lockdown that resulted in the death of Tamerlan Tsarnaev, 26, and the capture of his 19-year-old brother, Dzhokhar. 

“All in all, this has been a tough week,” said President Barack Obama, addressing the nation Friday night. “But we’ve seen the character of our country once more.”

Through its long history, America has weathered its share of the disturbing and the traumatic -- political assassinations, civil and international wars, school massacres, Pearl Harbor, the 9/11 attacks. But few in this generation can cite a single Monday-through-Friday series so jam-packed with frightful, breaking-news bulletins.

“For the first time in a long time, we’re really being challenged now on our home turf,"  said Marleen Wong, a professor and associate dean of the University of Southern California school of social work. She compared the condensed spate of sadness to the 1960s assassinations of President John Kennedy, his brother Robert, and Martin Luther King, though she admits those murders spanned five years.

Sure, it's a lot to take. But when do we hit our bad-news breaking point? 

"If there’s another IED in another city, then we’re really going to have a problem. That’s what concerns me. We might then be crossing some kind of new line," said Bart Rossi, a New Jersey psychologist and author of "The New-New American Life Style: Post September 11, 2001, A Psychologist’s Perspective." "We're talking about some heavy issues here." 

Already, he expects that many Americans are purposely avoiding crowds and staying home, fearful that another mass-casualty is looming. He estimates that in about one month, those same people will resume their normal routines — if all remains relatively quiet.

"If you put a number on our national anxiety it's a 6 or 7 or maybe trending toward an 8," Rossi said. "We’re so frustrated and angry. If something else happens, it might go up to a 9 or a 10, where we’re all just really overwhelmed and overwrought."

That’s true even though the actual risk of harm is very small, even for those who were confined in the immediate area of Watertown, Mass., where Dzhokhar Tsarnaev was believed to be trapped for most of Friday.

“The risk is statistically infinitesimal,” said David Ropeik, a Harvard University instructor, author and consultant in risk communication. “And greater emotionally.”

Terrorism is effective precisely because of the emotions it evokes and the stress that triggers a flight-or-fight response that suppresses reason and makes people more instinctive, Ropeik said.

“What terrorism is, is random, violent madness that makes us all feel vulnerable,” he said. “The unpredictable, unpreventable, could-happen-to-anyone-anywhere-anytime, they-are-living-among-us crimes always scare us.”

And it's not like Americans have been dancing lately through a landscape of easy years. The nation has weathered two wars — one still active — and the nasty aftermath of those conflicts, a bad economy, and an adversarial political environment: not traumatic for most yet exhausting and grinding for many. Since last summer, we've mourned dozens lost in the Aurora theater massacre, Superstorm Sandy and the Newtown school slaughter.

"These are times that really reinforce our values and the things we hold dear: the ability to live in peace," Wong said. 

"But on the other hand, I hear messages not just from leaders but also from people, from athletic teams, from runners — from people who have expressed the idea that you can try to hurt Americans, but we’re not afraid, we’re going to respond, we’re going to keep going, we’re going to prevail.

"It really demonstrates the courage of Americans in a way that reminds me of Britain during World War II when the bombs were falling every day in London and their leader, Winston Churchill, stood up and described what the English spirit is all about," Wong said. In similar fashion, some have demonstrated heroic and defiant actions this week — like the Boston hockey crowd belting out the National Anthem on Wednesday night.

"I saw that. It was so wonderful. It made me cry," Wong said. "We will be together, and we’ll get through it."

Related stories:

• 'We got him!' Boston bombing suspect captured alive
• Massive Boston manhunt drags on; anxiety grips city
• Who are the Tsarnaev brothers?

Joe Raedle / Getty Images, file

As the U.S. Senate takes up gun legislation in Washington, DC , Mike Acevedo puts a weapon on display at the National Armory gun store on April 11, 2013 in Pompano Beach, Florida.

A Senate proposal to expand criminal background checks to people who buy firearms at gun shows and online would increase sales at traditional gun stores, many retailers agree — and perhaps even hand licensed dealers a “sweetheart” boon that amounts to “an Obama tax,” according to one industry leader.

The bipartisan plan to broaden background checks — fueled by anger from the mass shooting in Newtown, Conn., as well as the gun-control push from the Obama Administration — would “bring a lot of money” to the bricks-and-mortar gun sellers, predicts Andrew Molchan, director of the National Association of Federally Licensed Firearms Dealers, which has about 1,000 members.

“It's kind of like a sweetheart union deal" that would be realized by gun store owners Molchan said. “Most FFL holders already charge for [private gun] transfers, and when it becomes a law they'll charge more." 

If instant criminal background investigations were to be federally mandated for all guns sold via the Internet or at gun shows, that task would fall to retailers. Some gun-store owners argue such a change would increase their workload and their legal risk, thus dampening the positive impact of having more firearms owners visit their establishments. But Molchan contends the tightened rules will ultimately deepen the revenue stream for licensed dealers.

“It's an ‘Obama Tax,’ with all of the money going to the FFL holders: gun stores, pawn shops, sporting goods stores, hardware stores,” he added. “The bottom line for the real world is that a year from now [if the law passes] there will be more gun stores.”

Other firearms sellers and industry analysts don’t envision gun-shop cash registers humming at quite the rapid pace that Molchan forecasts should Congress vote to pass the bill, but there seems to be consensus that profits at those locations will rise to some degree. 

'Treacherous direction'
An earlier plan pitched by the White House to require universal background checks — to cover all private firearms sales — would have generated an even larger payday for gun shops through far heavier foot traffic and even fatter bumps in side sales of ammunition and cleaning supplies, said Garen Wintemute, a firearms researcher and a professor at the University of California, Davis, where he also serves director of the Violence Prevention Research Program.

“The current [Senate] proposal falls well short of a comprehensive background-check policy [so] the benefit to retailers will be smaller than it otherwise will be,” Wintemute said.

Advocates for tougher gun laws have long contended that 30 to 40 percent of criminal firearm acquisitions are made from family and friends and, thus, done off the books, without background checks. However, many gun owners and sellers argue that number was plucked from 1990s research and that actual portion is probably closer to 10 to 13 percent. 

Gun shows typically include many tables occupied by licensed sellers (including retailers) who are required by law to conduct background checks even at those transitional venues. That means the proposed background-check extension would only affect “individuals who are selling their personal collections” at such events, and “that’s not a big factor,” said Larry Hyatt owner of Hyatt Gun Shop in Charlotte, N.C.

“One reason a lot of people want to buy a gun at a gun show from another individual is because they don’t want the government to know. They’re not buying because they are criminals or have criminal intent. They just want to be invisible,” Hyatt said. “That’s a huge issue in the country."

Broadening background checks to include gun shows and Internet transactions will have “a pretty small” impact on stores like Hyatt’s, he said. “There are still going to be people who wouldn’t buy from us anyway because they wouldn’t want a record [of the purchase]. It’s not anything evil. It’s brought about by some of this fear of government and fear of future gun laws. Because people see an inevitable descent [toward gun bans]. They see an overall treacherous direction.”

W.V. State Police via Reuters

Tennis Melvin Maynard, 37, is seen in this undated handout photo released by the West Virginia State Police.

A man alleged to have shot and killed a West Virginia sheriff on April 3 should have been barred from owning a gun, but got his hands on a weapon after his background check was delayed by "kinks in the chain" a county prosecutor said.

Mingo County Sheriff Eugene Crum was sitting in his parked police SUV eating lunch when Tennis Melvin Maynard, 37, allegedly shot him twice using a .40 caliber Glock handgun, police have said.

"It was a federal and state violation for him to possess a firearm, and he possessed other firearms also," Mingo County Prosecuting Attorney C. Michael Sparks told NBC News. Sparks declined to say what on Maynard's record prohibited him from owning the gun with which he allegedly shot Crum.

"The dealer did what was legally required under the law," Sparks said. "The disqualifying event ... it was not in the federal database when the gun was purchased. There was a delay in the time period between the triggering event and the information being reported to the federal database."

A separate, subsequent attempt by Maynard to buy a firearm failed when the background check system flagged him, Sparks said.

Sparks said West Virginia has “one of the more sophisticated systems in America as far as reporting this type of information.”

Maynard fled from the alleged shooting, police said, but was stopped when his car crashed into a bridge. After raising his gun to a pursuing deputy, Maynard was shot. He was transported to a hospital and authorities have said he is recovering from his injuries. Maynard has been charged with murder and attempted murder.

Williamson Daily News via AP

This undated photo shows Mingo County Sheriff Eugene Crum. Crum was gunned down Wednesday, April 3, 2013.

Maynard had spent time in a mental institution and “the same problem was eating him again,” his father told The Associated Press. Federal law prohibits the sale of guns to people who have been adjudicated mentally defective or spent time in an institution.

“He would have probably shot anybody, the first one he come to, you know what I’m saying,” Maynard's dad, Melvin, said. “I know he was off, I know he should have been in a hospital.”

A funeral for Crum, 59, at Mingo Central High School on Sunday was attended by close to 400 law enforcement officers who remembered the sheriff for his efforts to combat Mingo County’s drug trade.

“We ask all the time where have all the heroes gone?” Mingo County Circuit Judge Michael Thornsbury said in a eulogy, according to the AP. “Let me tell you, sometimes we walk in their midst and we don’t know we got them. He was mine.”

Crum’s wife, Rosie, was appointed to fill her husband’s position as interim sheriff on April 4. The county’s first female sheriff, she was sworn in during a candlelight vigil honoring her husband.

The news that Maynard never should have been able to buy a gun came Wednesday as Senator Joe Manchin of West Virginia, a Democrat, proposed a bipartisan deal with Sen. Patrick Toomey that would expand background checks and strengthen the National Instant Criminal Background Check System (NICS) by refusing some federal funds to stats that fail to submit full records. The NICS was established in 1993 by the Brady Handgun Violence Prevention Act.

Randy Snyder / AP

Members of the honor guard carry the body of the late Mingo County Sheriff Eugene Crum on Sunday, April 7, 2013, at the Mingo Central High School in Matewan, W.Va.

Related:

• Dad: W. Va. sheriff slaying suspect mentally 'off'
• West Virginia sheriff slain while eating lunch in car
• West Virginia sheriff shot dead, suspect wounded

Two critical senators with “A” ratings from the National Rifle Association proposed a deal Wednesday that would expand background checks on firearms sales, which are currently required on purchases from federally licensed dealers. The compromise proposal put forward by Senators Joe Manchin and Pat Toomey would mandate them for sales at gun shows and on the Internet as well, yet make an allowance for transfers between family members.

More than 167 million checks were made through the FBI's system between 1998 and early 2013, but the process remains obscure to many Americans. What are background checks, and why has it taken so long for lawmakers to piece together a deal on a measure polls say is overwhelmingly favored by American voters? Here’s a primer:

How do background checks work now?

The Brady Handgun Violence Prevention Act of 1993 established the National Instant Criminal Background Check System (NICS), which allows the seller to check a buyer’s eligibility with a search that usually takes less than a minute. The system was fully launched in 1998. Before selling a gun, the gun store worker calls in to the FBI or other designated law enforcement agency to run a check against the system’s records. If the prospective buyer’s record doesn’t raise a red flag – possible triggers include a person having been adjudicated as mentally ill or being sought by law enforcement – the sale is cleared to go through.

What kinds of gun purchases don’t require background checks under current law?

That depends on where you live. In the wake of the Newtown school shooting, President Obama asked for a federal law that would require universal background checks, including at gun shows. Right now, only California, Colorado, Illinois, New York, Oregon, and Rhode Island require background checks at gun shows, according to the Coalition to Stop Gun Violence. But most states have looser restrictions. While local laws can vary widely, 33 states do not have a law addressing what is commonly referred to as the “gun show loophole.” Similarly, regulations on sales between private parties or transfers between family members can be very different from state to state, where they exist at all.

Is the background-check system foolproof?

Critics of the current background check system point to gaping holes in the ways states submit records to the NICS. While 44 states have individual laws regulating the sale of firearms to the mentally ill, for example, far fewer states submit the names of prohibited mentally ill individuals to the national database. Just seven states account for 98 percent of the names prohibited for mental illness, according to Mayors Against Illegal Guns, meaning most states are in there barely, if at all. In one oft-cited example, Virginia Tech shooter Seung-Hui Cho passed a background check before obtaining a gun and killing 32 people, despite having been declared mentally ill two years before. States are responsible for compiling mental health records from courts, hospitals, and other sources to submit to NICS, but they are not legally required to do so.

Does the public support broader background checks?

The vast majority of American voters do. Eighty-five percent of Americans said they support background checks at gun shows and for private sales in a Pew Research Center poll released earlier this year. Other polls have found even wider support for broadening checks, with 92 percent of respondents to a February survey by Quinnipiac University saying they favored them on every single gun sale. That number dropped to 91 percent among gun-owning households.

Given this level of support, why aren’t universal background checks already law?

That’s a harder question to answer, as the issue becomes bitterly political. Momentum on Capitol Hill toward a bill requiring comprehensive background checks has been slow to gain traction. Republican Senators Ted Cruz, Rand Paul, and Mike Lee said that they would filibuster debate on new gun legislation, but that idea lost steam on Tuesday as other Republican lawmakers including Sen. John McCain said they would not support a filibuster. The NRA released a statement on Wednesday after the Manchin-Toomey compromise was announced saying that expanding background checks “will not prevent the next shooting, will not solve violent crime and will not keep our kids safe in schools.” Other opponents of expanded background checks have argued that they would require a national registry of gun owners, something the White House has denied.

Are background checks effective?

The numbers show that background checks do keep guns out of the hands of at least some people who are not supposed to have them. Nearly 1.8 million applications for firearm transfers or permits were denied between the passage of the law in March 1994 and December 2008, according to the Bureau of Justice Statistics. The FBI and state law enforcement denied firearm purchases to 153,000 people in 2010 alone, the most recent year for which data is available.

Jim Lo Scalzo / EPA file

Shoppers examine handguns on display for sale at The Nation's Gun Show held in the Dulles Expo Center in Chantilly, Virginia, USA, 28 July 2012.

Related:

• 'The monster want out:' Mentally ill killer amassed huge arsenal, police say
• Support soars for tougher gun laws, surveys show
• Gun instructor could get permit back after threatening to 'start killing people'