An investigation of the FBI's handling of the events leading up to the shootings at Fort Hood, Texas, in November 2009, concludes that agents made a series of mistakes, failing to follow up on important questions and to share information widely enough.

"We do not find, and we do not suggest, that these mistakes resulted from intentional misconduct or the disregard of duties," concluded William Webster, the FBI's former director who led the investigation. "Indeed, we find that each special agent, intelligence analyst, and task force officer who handled the information acted with good intent."

Click here to read the full report (pdf)

Most of the shortcomings have been previously disclosed, and some resulted from a lack of training and of understanding military nomenclature. For example, agents in San Diego, who were investigating al-Qaida propagandist Anwar al-Awlaki, noticed on December 17, 2008, that Nidal Hasan, who would become the Fort Hood shooter, sent al-Awlaki an e-mail asking about soldiers who kill fellow military personnel with the aim of "helping muslims fighting jihad."

Related: Judge delays Fort Hood shooting hearing over Hasan's beard

The San Diego agents decided against sending out a broadly disseminated message that would have alerted the system that a member of the US military was communicating with a known al-Qaida terrorist. The agents noticed that a summary of his military records said Hasan was a "Comm Officer," and they assumed it meant he was a communications officer and might have access to the system that would contain such an alert message. In fact, the abbreviation meant Hasan was a commissioned officer.

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The report also says agents in the FBI's Washington field office failed to follow through more aggressively to the leads developed in San Diego. Part of the problem, the report said, was that the FBI received only glowing accounts from the Department of Defense about Hasan's career. Agents were never told that he was actually considered a poor performer who was often on probation.

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The U.S. government is legally justified in killing its own citizens overseas if they are involved in plotting terror attacks against America, Attorney General Eric Holder said Monday, offering the Obama administration's most detailed explanation so far of its controversial targeted killing program.

"In this hour of danger, we simply cannot afford to wait until deadly plans are carried out, and we will not," he said in remarks prepared for a speech at Northwestern University's law school in Chicago.

An American-born Islamic cleric, Anwar al Awlaki, was killed in a U.S. drone attack in Yemen in late September.  Some civil liberties groups condemned the attack. Others, including members of Congress, called for a more complete explanation of how such a targeted killing of an American civilian was consistent with the U.S. Constitution.

The Fifth Amendment provides that no one can be "deprived of life" without due process of law.  But that due process, Holder said, doesn't necessarily come from a court.

"Due process and judicial process are not one and the same, particularly when it comes to national security.  The Constitution guarantees due process, not judicial process," the attorney general said.

Holder said a U.S. citizen can legally be targeted in a foreign country if that person is "a senior leader of al-Qaida or associated forces," and is actively involved in planning to kill Americans.  Killing would be justified if the person poses an imminent threat of a violent attack against the U.S. and cannot easily be captured.

Any military operation targeting a citizen overseas must be carried out consistent with the law of war.  "The principle of humanity requires us to use weapons that will not inflict unnecessary suffering," he said.

The ACLU called Holder's explanation "a defense of the government’s chillingly broad claimed authority to conduct targeted killings of civilians, including American citizens, far from any battlefield without judicial review or public scrutiny."

"Few things are as dangerous to American liberty as the proposition that the government should be able to kill citizens anywhere in the world on the basis of legal standards and evidence that are never submitted to a court, either before or after the fact," said Hina Shamsi, director of the ACLU’s National Security Project.

"Anyone willing to trust President Obama with the power to secretly declare an American citizen an enemy of the state and order his extrajudicial killing should ask whether they would be willing to trust the next president with that dangerous power,” she said.

The ACLU is suing the Obama administration, seeking to have documents regarding the targeted killing program made public.

Holder said it makes no legal difference that a U.S. citizen is targeted away from a traditional battlefield.  "We are at war with a stateless enemy," he said.

While the U.S.-born cleric, Anwar al Awlaki, was at first believed to be merely an English speaking propagandist for the Yemen based group known as Al Qaeda in the Arabian Peninsula, U.S. officials say he gradually assumed an operational role in the terror group.

According to federal prosecutors, Umar Abdul Abdulmutallab, the underwear bomber, told FBI interrogators that al Awlaki was deeply involved in planning his attempted bombing of a Detroit bound passenger jet on Christmas day in 2009. 

Holder said Abdulmutallab told the FBI of "al Awlaki's specific instructions to wait until the airplane was over the United States before detonating the bomb."

The attorney general told the law students that the government is under no legal obligation to delay a targeted killing operation until a terrorist plotter is in the process of carrying out an actual attack.

"The Constitution does not require the president to delay action until some theoretical end stage of planning, when the precise time, place, and manner of an attack become clear," he said.

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AP file

Umar Farouk Abdulmutallab has admitted trying to detonate an explosive device on a Dec. 25, 2009, flight from Amsterdam to Detroit.

Al-Qaida leader Anwar al-Awlaki personally directed and approved the attempted bombing of a U.S. airliner that a Nigerian man tried to carry out on Christmas 2009, according to new details released by federal prosecutors on Friday.

Awlaki was a leader of al-Qaida in the Arabian Peninsula, the militant group's affiliate in Yemen, before he was killed in a drone strike last year. Awlaki directed Umar Farouk Abdulmutallab to carry out a strike aboard an American airliner over U.S. soil, prosecutors say.

"Awlaki's last instructions to him were to wait until the airplane was over the United States and then to take the plane down," according to court papers. Awlaki left it up to Abdulmutallab to pick the flight and date, the papers said.

Abdulmutallab, 25, is due to be sentenced Thursday in Detroit and faces up to life in prison after pleading guilty to charges he tried to down a Northwest Airlines jumbo jet with 289 people aboard on a flight from Amsterdam to Detroit.

The bomb, hidden in his underwear, failed to fully detonate and he was subdued. The incident led U.S. security officials to quickly bolster airport security, deploying full-body scanners to try to detect explosives hidden in clothing.

In urging the federal court to impose a life sentence on Abdulmutallab, federal prosecutors Friday revealed new details about his contact in Yemen with Awlaki.

“This removes any doubt that Anwar al-Awlaki was directing this operation from soup to nuts,” a White House official said Friday, according to NBC News,  Some civil libertarians have said the U.S. lacked legal authority to kill Awlaki, who was a U.S. citizen.

“Congress has clearly granted the authority to target U.S. citizens who take up arms against the United States,” the official said. 

Among the details, according to prosecutors:

Abdulmutallab followed Awlaki's online teachings for several years and went to Yemen in 2009 hoping to arrange a meeting. He visited mosques and asked people he met if they knew how he could meet Awlaki, eventually finding someone who offered to help.

He received a text message from Awlaki and the two later talked by phone. After sending a long message about why he wanted to engage in jihad, Abdulmutallab was cleared to meet Awlaki and was driven through the desert to Awlaki's house, where he stayed for three days, discussing martyrdom. 

He was then driven to another house, where he met Ibrahim al-Asiri, the bomb maker for al-Qaida in Yemen. They talked about a plan for the terrorism mission, and Abdulmutallab was trained in al-Qaida camp.

Al-Asiri made the underwear bomb, delivered it to Abdulmutallab, and told him how to detonate it -- by pushing the plunger of a syringe that would mix two chemicals, starting a fire, and setting off the main explosive charge.

Abdulmutallab recorded a five-minute martyrdom video and left Yemen, with the authority to choose the flight and date to attack. 

In October, Abdulmutallab pleaded guilty days after his trial began, saying he commited the crime because he wanted to avenge the killing of innocent Muslims by the United States.

In a sentencing memorandum filed in federal court in Detroit, prosecutors urged a judge to sentence Abdulmutallab to the maximum of life in a U.S. prison.

Obama administration officials said the information about Abdulmutallab’s activities in Yemen were obtained from his interrogation by an FBI agent.

“This demonstrates that it’s not necessary to charge terrorists as enemy combatants and put them before military tribunals in order to gain valuable intelligence,” a White House official said Friday.

NBC News Chief Justice Correspondent Pete Williams and Reuters contributed to this story.

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