As a medical student, he reportedly required counseling because of issues with patients.  He was under federal observation for possible involvement with radical internet postings.  Some say he fought his deployment to Iraq.

Nidal Malik Hasan is the man authorities say is responsible for the worst mass killing on a U.S. military base.  But his motive remains a mystery.

Before reporting to Fort Hood, Hasan worked for six years at the Walter Reed Army Medical Center where he had a career in psychiatry as an intern, resident and last year as a fellow in disaster and preventative psychiatry.   The 39-year-old Army major received his medical degree from the military's Uniformed Services University of the Health Sciences in Bethesda, Md., in 2001.

According to Dr. Thomas Grieger, a training director at Walter Reed, Hasan had some "difficulties" that required counseling and extra supervision.

A few months ago, federal law enforcement became interested in Hasan because of Internet postings about suicide bombings and other threats, including posts that equated suicide bombers to soldiers who throw themselves on a grenade to save the lives of their comrades.

They had not determined for certain whether Hasan is the author of the posting, and a formal investigation had not been opened before the shooting, according to officials.

One of the officials said late Thursday that federal search warrants were being drawn up to authorize the seizure of Hasan's computer.

Retired Army Col. Terry Lee, who said he worked with Hasan, told Fox News that Hasan had hoped President Barack Obama would pull troops out of Afghanistan and Iraq. Lee said Hasan got into frequent arguments with others in the military who supported the wars, and had tried hard to prevent his pending deployment.

Hasan earned his rank of major in April 2008, according to a July 2008 Army Times article.

He served eight years as an enlisted soldier. He also served in the ROTC as an undergraduate at Virginia Tech University in Blacksburg. He received a bachelor's degree in biochemistry there in 1997.