Days after a mass shooting at the Fort Hood Army post in Killeen, Texas, details of the gunman's life have captivated millions looking for motives behind Maj. Nidal Malik Hasan's murderous rampage.
As military psychiatrist trained to counsel troops returning from combat, Hasan's personal history has sparked many theories about why he turned on the very people he was employed to counsel -- killing 13 and wounding at least 30.
Some see this as an act of terrorism, but crime experts and fellow psychiatrists familiar with the military question whether Hasan's alleged actions compare with those of George Sodini, who is accused of shooting 11 women in a Pennsylvania gym this summer, or Seung-Hui Cho 's motives in the 2007 Virginia Tech Massacre.
"What went wrong?" asked Dr. Paul Ragan, a former military psychiatrist and now a professor at Vanderbilt University. "I keep thinking of the psychology that we know about mass shooters -- the Texas clock tower shooter from 1966, Columbine and Virginia tech," he said.
According to Ragan, profiles of a mass murderer often cluster into age groups of teens and early 20s, "or they seem to be men in their early- to mid-40s -- disaffected in their careers, many of them are loners, they have poor social skills and poor interpersonal skills."
Mass murderers tend to come in two types, according to academic articles authored by forensic psychologist J. Reid Meloy. One type is predatory, premeditated and emotionless. The other acts out from anger, fear, or response to a perceived imminent threat or trigger.
Hasan is an American of Palestinian decent. Born and raised in Virginia, he went on to Virginia Tech and joined the U.S. Army to become a psychiatrist. His aunt, Noel Hasan of Falls Church, Va., told the Washington Post that he was single and had no girlfriend and that "he did not make many friends" and "did not make friends fast."
After Sept. 11, 2001, his aunt said Hasan was harassed about his Islamic faith and had sought for several years to be discharged from the military.
Dr. Amir Afkhami, an assistant professor at George Washington University, was shocked at the violence Thursday and surprised by the many parallels between his life and Hasan's -- both have a Muslim heritage, both grew up in Virginia and both work with the military as a psychiatrist.
Yet, Afkhami said, "I don't think we should get lost on his identity."
Family and neighbors have told ABC News that Hasan was a pious Muslim and solitary man, but was outspoken about his views against the current wars in Iraq and Afghanistan.
"He's a quiet man, he looked like a nice person to me, so since I have been living here I never heard a noise in that house," said Vivian Tchangan, his former neighbor in Silver Spring, Md. Tchangan added that Hasan wrote "Allah" on his door.
Hasan relocated to Fort Hood in Texas and was soon to be deployed to Afghanistan, U.S. Army officials confirmed Friday. He is now in critical condition and paralyzed at a hospital in Texas after being shot multiple times by a police officer.