If a larger plot wasn't being carried out, then what was the motive?
Military officials have not yet declared an intent or motivation. But even though the investigation is still in the early stages, a more focused portrait is emerging of Hasan, who is recovering in a San Antonio hospital. Security and criminal analysts are hinting that belligerent fanaticism, deepening anger over the wars in Iraq and Afghanistan, and fear about his own impending deployment to Afghanistan may have played a role in the rampage.
Dozens of investigators are building a psychological profile of Hasan as they try to understand the motive. This profile probably won't be completed until Hasan is able to, and agrees to, talk. (He is now in stable condition, according to a US Army spokesman, and is breathing on his own.)
On Sunday, the US Army's chief of staff cautioned against drawing hasty conclusions about Hasan's faith. Focusing on the Islamic roots of Hasan could "heighten the backlash" against all Muslims in the military, said Gen. George Casey, appearing on NBC's "Meet the Press."
Hasan grew up as a US-born son of Palestinian parents. He had sought deeper sanctuary in his Muslim faith since the death of his parents, his cousins have reported. His psychological work with traumatized US soldiers had disturbed him.
Without an immediate family and increasingly disenchanted with the Army, Hasan turned to religion, where he could direct his increasing fear and frustration, says Brian Levin, director of the Center for the Study of Hate & Extremism at California State University, San Bernardino.
"The spark was his personal situation and his psychological distress, but what directed his anger and frustration was ... increasing self-radicalization," Mr. Levin says. "It's important to understand the extent to which the tangled interplay of personal disappointment and ideology can have on someone like Hasan."
After arriving earlier this year at the base city of Killeen, Texas, Hasan asked the local imam about the moral quandary he struggled with – as a Muslim going off to war in an army that is fighting Muslims.
Hasan had complained about being the target of religious and ethnic slurs, his family and colleagues said, but he never made an official complaint with the Army.
According to a Washington Post account, he once called the police on a neighbor for allegedly keying his car and removing a bumper sticker that said in Arabic, "Allah is love."
Colleagues reported that Hasan had become increasingly agitated, sometimes belligerent, about America's role in the Muslim world. A former colleague told Fox News that Hasan had praised the shooting of two Army recruiters by a "lone wolf jihadist" earlier this year in Little Rock, Ark. His online writings had gotten the attention of the Federal Bureau of Investigation, which investigated but took no action against Hasan.
Last Thursday, Hasan reportedly yelled "Allahu Akbar" – "God is great" in Arabic – before allegedly opening fire on soldiers in various stages of deployment and return at Fort Hood's Soldier Readiness Processing Center.
That the Army had not picked up on clues from Hasan – in fact, it even promoted him to major last year despite some negative marks on his record – is likely to become a secondary line of inquiry in the Hasan case.
On Sunday, Sen. Joseph Lieberman (I) of Connecticut announced that he wants to lead a congressional investigation into the Fort Hood shootings. He wants to find out "whether the Army missed warning signs" about Hasan, he said on "Fox News Sunday."
One potential issue: Did political correctness around religious issues prevent Army officials from investigating Hasan's issues more deeply?
"His faith may have exposed him to verbal abuse, but did it also protect him from being identified as someone with troubling emotional problems?" asks the columnist Joan Smith in Britain's Independent newspaper.] "[I]t is reasonable to ask whether secular authorities have the confidence to tell the difference between religious fervour of a kind they're not familiar with and genuinely disturbed behaviour."
As of now, it appears to many that Hasan may have been unable to reconcile two identities – US soldier and a Muslim – and that the two identities clashed in violent opposition as he approached a major personal landmark – his own deployment to a war zone that he had seen create challenges for so many young people.
For many at Fort Hood, though, those are just psychobabble excuses. "To be honest, I don't care about that guy right now," says Third Corps Sgt. Christopher Gray.
Follow us on Twitter.