FORT HOOD, Texas (AP) – A military mental health doctor facing deployment overseas opened fire at the Fort Hood Army post on Thursday, setting off on a rampage that killed 11 other people and left 31 wounded. Authorities killed the gunman, and the violence was believed to be the worst mass shooting in history at a U.S. military base.

The shooting began around 12:30 p.m. when shots were fired at the base’s Soldier Readiness Center, where soldiers who are about to be deployed or who are returning undergo medical screening, said Lt. Gen. Bob Cone at Fort Hood.

President Barack Obama called the shooting “a horrific outburst of violence.” He said it is a tragedy to lose a soldier overseas and even more horrifying when they come under fire at an Army base on American soil.

“We will make sure that we get answers to every single question about this horrible incident,” the commander in chief said in Washington. “We are going to stay on this.”

A law enforcement official identified the shooting suspect as Army Maj. Nidal Malik Hasan. The official spoke on condition of anonymity because he was not authorized to discuss the case publicly.

It was unclear what the motive was, though it appeared he was upset about a scheduled deployment. U.S. Sen. Kay Bailey Hutchison said the Army major was about to deploy overseas, though it was unclear if he was headed to Iraq or Afghanistan and when he was scheduled to leave. Hutchison said she was told about the upcoming deployment by generals based at Fort Hood.

Retired Army Col. Terry Lee told Fox News that he worked with Hasan, who had hoped 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.

Military officials say Hasan, 39, was a psychiatrist at Walter Reed Army Medical Center for six years before being transferred to the Texas base in July. The officials, who had access to Hasan’s military record, 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 graduated from Virginia Tech, 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, Maryland, in 2001. At Walter Reed, he did his internship, residency and a fellowship.

Officials were investigating whether Hasan was his birth name or if he may have changed his name, possibly as part of a conversion to Islam. However, they were not certain of his religion.

The Soldier Readiness Center holds hundreds of people and is one of the most populated parts of the base, said Steve Moore, a spokesman for III Corps at Fort Hood. Nearby there are barracks and a food center where there are fast food chains. The center is part of the largest active duty armored post in the United States. Covering 339 square miles, the post halfway between Austin and Waco was home to about 52,000 troops as of earlier this year.

A graduation ceremony for soldiers who finished college courses while deployed was going on nearby at the time of the shooting, said Sgt. Rebekah Lampman, a Fort Hood spokeswoman.

Soldiers don’t carry weapons with them unless they’re doing training exercises, said Spc. Jerry Richard, 27, who works at the building where the shooting happened – though he was not on post at the time.

“Overseas you are ready for it. But here you can’t even defend yourself,” he said.