Doctors who crossed paths with Hasan in medical programs paint a picture of a subpar student who wore his religious views on his sleeve.
Several doctors who knew Hasan spoke to CNN, but only on condition of anonymity because of the ongoing investigation of the shooting, which left 12 soldiers and one civilian dead and dozens of other people wounded.
Hasan, an Army psychiatrist who faces 13 counts of premeditated murder, "was clearly espousing Islamist ideology" during his time as a medical student at the Uniformed Services University of the Health Sciences in Bethesda, Maryland, one of his former classmates told CNN.
Hasan's family has revealed little about him, saying in media interviews that Hasan was a "good American" and a lifelong Muslim who complained he was harassed in the Army because of his religion.
His former classmates describe a much more militant Hasan.
His presentations for school were often laced with extremist Muslim views, one source said.
"Is your allegiance to Sharia law or the United States?" students once challenged Hasan, the source said.
"Sharia law," Hasan responded, according to the source.
The incident was corroborated by another doctor who was present.