Hasan, who was reportedly upset about an upcoming deployment to Iraq, had been disciplined in the past for preaching to his patients about his Muslim faith.
Earlier in the day, Hasan had cleaned out his apartment, gave leftover frozen vegetables to a neighbor and called another to thank him for his friendship, the Associated Press reports.
Jacqueline Harris, 44, who lives with her boyfriend Willie Bell in the apartment next door to Hasan, told the AP that the alleged shooter had left a phone message on Thursday at 5 a.m.
Bell said Hasan had said it was "nice knowing you old friend. I'm going to miss you."
Hasan's cousin, Nader Hasan, issued a statement late Thursday.
"We are shocked and saddened by the terrible events at Fort Hood today. We send the families of the victims our most heartfelt sympathies," the statement read. "Nidal was an American citizen. He was born in Arlington, Va., and raised here in America. ... Our family loves America. We are proud of our country, and saddened by today's tragedy."
Hasan had reportedly recently hired an attorney to help him get out of the military.
According to the suspect's cousin, Hasan was also harassed after 9/11 because of his ethnicity, and was called a "camel jockey."
According to sources, Hasan, who is 39, attended the Uniformed Services University of the Health Sciences in Bethesda, Md, and later finished his residency as a psychiatrist.
In 2009, Hasan completed a fellowship in disaster and preventive psychiatry at the Center for Traumatic Stress.
He was promoted to major status in May, according to the Army Times.
According to Tthe Associated Press, retired Army Col. Terry Lee told Fox News that Hasan had expressed hope that Obama would pull troops out of Afghanistan and Iraq, and often argued with others who supported the wars.
ABC News' Ethan Nelson and Desiree Adib contributed to this report.