The outburst came during an hour-long talk Hasan, an Army psychiatrist, gave on the Koran in front of dozens of other doctors at Walter Reed Army Medical Centre in Washington DC, where he worked for six years before arriving at Fort Hood in July.
Colleagues had expected a discussion on a medical issue but were instead given an extremist interpretation of the Koran, which Hasan appeared to believe.
Hasan, armed with two handguns including a semi-automatic pistol, walked into a processing centre for soldiers deploying to Iraq and Afghanistan, where he killed 13 and injured more than 30.
Fellow doctors have recounted how they were repeatedly harangued by Hasan about religion and that he openly claimed to be a "Muslim first and American second."
One Army doctor who knew him said a fear of appearing discriminatory against a Muslim soldier had stopped fellow officers from filing formal complaints.
Another, Dr Val Finnell, who took a course with him in 2007 at the Uniformed Services University of the Health Sciences in Maryland, did complain about Hasan's "anti-American rants." He said: "The system is not doing what it's supposed to do. He at least should have been confronted about these beliefs, told to cease and desist, and to shape up or ship out. I really questioned his loyalty."
Selena Coppa, an activist for Iraq Veterans Against the War, said: "This man was a psychiatrist and was working with other psychiatrists every day and they failed to notice how deeply disturbed someone right in their midst was."
One of Hasan's neighbours described how on the day of the massacre, about 9am, he gave her a Koran and told her: "I'm going to do good work for God" before leaving for the base.
A civilian police officer who shot him, bringing the rampage to an end, said Hasan appeared "calm" during the massacre, hiding behind a telephone pole and shooting fellow soldiers in the back as they tried to get away.
"He was firing at people as they were trying to run and hide, said Sgt Mark Todd. "Then he turned and fired a couple of rounds at me. I didn't hear him say a word, he just turned and fired."
Hasan flinched after he was shot and slid down against the pole still clutching his gun, which had a laser sight on it. The officer kicked away the weapon and handcuffed him.
He said: "The guy was breathing, his eyes were blinking. I could tell that he was fading out and he didn't say anything. He was just kind of blinking."
Senator Joe Lieberman, who chairs the US Senate Committee on Homeland Security, said there had been "strong warning signs" that Hasan was an "Islamist extremist".
The committee would ask "whether the Army missed warning signs that should have led them to essentially discharge him, he said. He added: "The US Army has to have zero tolerance. He should have been gone."
But General George Casey, the Army's Chief of Staff, said it was "speculation" that military authorities failed to pick up on warning signs. "I don't want to say that we missed it," he said.
Asked if military authorities had missed warning signs Gen Casey, the Army's Chief of Staff, added: "We have to go back and look at ourselves ,and ask ourselves the hard questions. Are we doing the right things? We will learn from this.
"It's too early to draw conclusions but we will ask ourselves the hard questions about what we are doing and the changes we should make as a result of this."