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

There are many unknowns about Major Nidal Malik Hasan, the man authorities say is responsible for the worst mass killing on a US military base. Most of all, his motive.

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, Maryland, in 2001.

While an intern at Walter Reed, Maj Hasan had some “difficulties” that required counseling, said Thomas Grieger, who was the training director at the time.

Grieger recalled Maj 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, Maj 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. They had not determined for certain whether he is the author of the posting.

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