The Texas army camp massacre gunman was being investigated for possible links to an extremist website.
At least six months ago Major Nidal Malik Hasan came to the attention of law enforcement officials because of internet postings about suicide bombings and other threats.
They included posts that compared suicide bombers to soldiers who throw themselves on a grenade to save the lives of their comrades.
Investigators were not certain whether Hasan had written the posts and a formal investigation had not been opened before the shooting, said law enforcement sources.
Meanwhile, military officials were starting to piece together what might have pushed the psychiatrist trained to help soldiers in distress to turn on his comrades.
Authorities have not ruled out that Hasan was acting on behalf of some unidentified radical group.
Nor are they ruling out the possibility that some of the casualties may have been victims of "friendly fire," that in the mayhem and confusion at the shooting scene some of the responding military officials may have shot some of the victims.
Retired Colonel Terry Lee, who said he had worked with Hasan, told Fox News he was being sent to Afghanistan.
Col Lee said Hasan had hoped President Barack Obama would pull troops out of Afghanistan and Iraq and got into frequent arguments with others in the military who supported the wars.
For six years before reporting for duty at Fort Hood in July, the 39-year-old army major worked at the Walter Reed Army Medical Centre pursuing a 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.
But his record was not perfect. At Walter Reed, he received a poor performance evaluation, according to one official.
And while he was an intern, Hasan had some "difficulties" that required counselling and extra supervision, said Dr Thomas Grieger, who was the training director at the time.
Hasan attended prayers regularly when he lived outside Washington, often in his Army uniform, said Faizul Khan, a former imam at a mosque Hasan went to in Silver Spring, Maryland. He said Hasan was a lifelong Muslim.
"I got the impression that he was a committed soldier," Mr Khan said.
On a mosque form Hasan listed his birthplace as Arlington, Virginia, but his nationality as Palestinian, Mr Khan said.
"I don't know why he listed Palestinian," Mr Khan said, "He was not born in Palestine."
Nothing stood out about Hasan as radical or extremist, he said.
"We hardly ever got to discussing politics," Mr Khan said. "Mostly we were discussing religious matters, nothing too controversial, nothing like an extremist."