Last Updated at 10:24 a.m. EST
Military officials were starting to piece together what may have pushed an Army psychiatrist trained to help soldiers in distress to turn on his comrades in a shooting rampage that killed 13 people and wounded 30 in Texas.
There are many unknowns about Nidal Malik Hasan, the man authorities say is responsible for the worst mass killing on a U.S. military base.
Hasan, who was shot four times during the rampage at the Army's sprawling Fort Hood, was on a ventilator and unconscious in a hospital, post officials said.
Federal authorities seized the suspect's computer and are looking for clues that may have led to the military massacre.
A U.S. law enforcement official said that Maj. Nidal Malik Hasan's apartment in Killeen, Texas, was searched early Friday. It was not immediately known if FBI agents found anything suspicious on Hasan's computer files.
A military official said investigators also are sifting through materials Hasan carried with him during the shooting incident and evidence left in his vehicle, which was found parked at the base.
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Fast Facts: Fort Hood
Tragedy at Fort Hood
For six years before reporting for duty at Fort Hood, Texas, in July, the 39-year-old Army major worked at the Walter Reed Army Medical Center pursuing his career in psychiatry, as an intern, a resident and, last year, a fellow in disaster and preventive psychiatry. He received his medical degree from the military's Uniformed Services University of the Health Sciences in Bethesda, Md., in 2001.
Hasan earned his rank of major in April 2008, according to a July 2008 Army Times article.
He served eight years as an enlisted soldier. He also served in the ROTC as an undergraduate at Virginia Tech in Blacksburg, where he received a bachelor's degree in biochemistry in 1997.
But his record wasn't sterling. At Walter Reed, he received a poor performance evaluation, according to an official who spoke on condition of anonymity because he was not authorized to discuss the case publicly. And while he was an intern, Hasan had some "difficulties" that required counseling and extra supervision, said Dr. Thomas Grieger, who was the training director at the time.
Grieger said privacy laws prevented him from going into details but noted that the problems had to do with Hasan's interactions with patients. He recalled Hasan as a "mostly very quiet" person who never spoke ill of the military or his country.
"He swore an oath of loyalty to the military," Grieger said. "I didn't hear anything contrary to those oaths."
But, more recently, federal agents grew suspicious.
At least six months ago, Hasan came to the attention of law enforcement officials because of Internet postings about suicide bombings and other threats, including posts that equated suicide bombers to soldiers who throw themselves on a grenade to save the lives of their comrades.
They had not determined for certain whether Hasan is the author of the postings, and a formal investigation had not been opened before the shooting, said law enforcement officials who spoke on condition of anonymity because they are not authorized to discuss the case.
One posting originated with an essay posted on Scribd.com, the popular document-sharing Web site, which argued that suicide bombings were not authorized by Islamic law. It said, for instance, that "it should be quite obvious that all of the 'evidences' used to justify suicide bombing by way of analogy are rather tenuous."
A subsequent, poorly-spelled comment from a user named "NidalHasan" posted in May 2009 seemed to take issue with that analysis. "If one suicide bomber can kill 100 enemy soldiers because they were caught off guard that would be considered a strategic victory," the comment said. "You can call them crazy i [sic] you want but their act was not one of suicide that is despised by Islam."
It's difficult to know from a single comment, of course, if that "NidalHasan" was the account used by the same person now in custody after the shooting. A public records search lists at least half a dozen people in the United States with the same first and last name.
Authorities have not ruled out that Hasan was acting on behalf of some unidentified radical group, a senior U.S. official in Washington said. He would not say whether any evidence had come to light to support that theory.
Retired Army Col. Terry Lee, who said he worked with Hasan, told Fox News that Hasan had hoped President Barack Obama would pull troops out of Afghanistan and Iraq. Lee said Hasan got into frequent arguments with others in the military who supported the wars, and had tried hard to prevent his own pending deployment.
A military official told The Associated Press that Hasan was in the preparation stage of deployment, which can take months. The official said Hasan had indicated he didn't want to go to Iraq but was willing to serve in Afghanistan. The official did not have authorization to discuss the matter publicly and spoke on condition of anonymity.
A cousin, Nader Hasan, told The New York Times that after counseling soldiers returning from Iraq and Afghanistan with post-traumatic stress disorder, Hasan knew war firsthand.
"He was mortified by the idea of having to deploy," Nader Hasan said. "He had people telling him on a daily basis the horrors they saw over there."
There have been reports that Hasan was harassed for his Muslim religion, but the official says there is no indication Hasan filed a complaint within the military about that.
Terrorism task force agents plan to interview several of Hasan's relatives Friday, according to a law enforcement official who spoke on condition of anonymity because the person was not authorized to discuss the case.
In an interview with The Washington Post, Hasan's aunt, Noel Hasan of Falls Church, Virginia, said he had been harassed about being a Muslim in the years after the Sept. 11, 2001, terror attacks and he wanted out of the Army.
"Some people can take it and some people cannot," she said. "He had listened to all of that and he wanted out of the military."
She said he had sought a discharge from the military for several years, and even offered to repay the cost of his medical training.
Hasan attended prayers regularly when he lived outside Washington, often in his Army uniform, said Faizul Khan, a former imam at a mosque Hasan attended in Silver Spring, Maryland. He said Hasan was a lifelong Muslim.
"I got the impression that he was a committed soldier," Khan said. He spoke often with Hasan about Hasan's desire for a wife.
On a form filled out by those seeking spouses through a program at the mosque, Hasan listed his birthplace as Arlington, Virginia, but his nationality as Palestinian, Khan said.
"I don't know why he listed Palestinian," Khan said, "He was not born in Palestine."
Nothing stood out about Hasan as radical or extremist, Khan said.
"We hardly ever got to discussing politics," Khan said. "Mostly we were discussing religious matters, nothing too controversial, nothing like an extremist."