Military officials say the suspected shooter,Maj. Nidal Malik Hasan, at Fort Hood was a psychiatrist at Walter Reed Army Medical Center for six years before being transferred to the Texas base in July.
The officials had access to Hasan's military record. They said he received a poor performance evaluation while at Walter Reed.
The officials spoke on condition of anonymity because military records are confidential.
The Virginia-born soldier was single with no children. He was 39 years old.
Federal law enforcement officials say Hasan had come to their attention at least six months ago because of Internet postings that discussed suicide bombings and other threats.
The officials say the postings appeared to have been made by Hasan, who was killed during the shooting incident that left least 11 others dead and 31 wounded. The officials say they are still trying to confirm that he was the author. They say an official investigation was not opened.
The officials spoke on condition of anonymity because they were not authorized to discuss the case.
One of the Web postings that authorities reviewed is a blog that equates suicide bombers with a soldier throwing himself on a grenade to save the lives of his comrades.
"To say that this soldier committed suicide is inappropriate. Its more appropriate to say he is a brave hero that sacrificed his life for a more noble cause," said the Internet posting. "Scholars have paralled (sic) this to suicide bombers whose intention, by sacrificing their lives, is to help save Muslims by killing enemy soldiers."
He is a graduate of Virginia Tech University, where he was a member of the ROTC and earned a bachelor's degree in biochemistry in 1997. He received his medical degree from the military's Uniformed Services University of the Health Sciences in Bethesda, Md., in 2001. At Walter Reed, he did his internship, residency and a fellowship.