Who was Nidal Malik Hasan?

His name appears on radical Internet postings. A fellow officer says he fought his deployment to Iraq and argued with soldiers who supported U.S. wars. He required counseling as a medical student because of problems with patients.

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. Most of all, his motive. But details of his life and mindset, emerging from official sources and personal acquaintances, are troubling.

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.

While an intern at Walter Reed, 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 posting, 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.

Federal law-enforcement agents ordered an evacuation of the apartment complex where Hasan lived in Killeen, Texas, Thursday night and conducted a search of his home, said Hilary Shine, director of public information for the city. She didn't say what was found during the search.

Officials said earlier that federal search warrants were being drawn up to authorize the seizure of his computer.

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 pending deployment.

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, Md. 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, Va., 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."

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 University in Blacksburg. He received a bachelor's degree in biochemistry there in 1997.

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