FORT HOOD, Texas - A military psychiatrist facing deployment overseas opened fire at the Fort Hood Army post yesterday, setting off on a rampage that killed a dozen people and left 31 wounded. The violence was believed to be the worst mass shooting in history at a U.S. military base.

The shooting began about 1:30 p.m., when shots were fired at the base's Soldier Readiness Center, where GIs who are about to be deployed or who are returning undergo medical screening, said the base commander, Lt. Gen. Bob Cone.

President 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.

Army officials identified the attacker as Nidal Malik Hasan, 39, a major who recently had been promoted from captain and worked at the Darnell Army Medical Center, Fort Hood's hospital.

Cone said last night that Hasan was not killed by authorities responding to the attack.

Officials said immediately after the attack that they had killed the suspected shooter. But Cone last night said Hasan was alive and under guard. Cone offered little explanation to reporters at a news conference as to why the suspect was believed to be dead, saying only there was confusion at the hospital.

Col. Ben Danner said the suspect was shot four times and was in critical condition.

"I would say his death is not imminent," Cone said.

It also was unclear what the motive was, though it appeared Hasan was upset about a scheduled deployment. U.S. Sen. Kay Bailey Hutchison said she had been told Hasan was about to deploy overseas, though it was unclear if he was headed to Iraq or Afghanistan and when he was scheduled to leave.

Faizul Khan, a former imam at a mosque Hasan attended in Silver Spring, Md., said he spoke often with Hasan about how Hasan wanted to find a wife. Hasan was a lifelong Muslim and attended prayers regularly, often in his Army uniform, Khan said.

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 several fast-food restaurants. The center is part of the largest active-duty armored post in the United States, halfway between Austin and Waco.

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.

The shooter used two pistols, one of them semiautomatic. Neither was military-issued, Danner said.

The rules for carrying weapons on an Army post are standard throughout all bases, service officials said. The only personnel allowed to openly display weapons on the base are military police. Service weapons are checked daily and are usually only allowed to be removed from an arms room for training on a range or maintenance. Personal weapons must be kept locked and registered with the base provost marshal. The military police keep a record of all of the weapons on a base, Army officials said.

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.

Two other soldiers taken into custody following the deadly rampage were released, Fort Hood spokesman Christopher Haug said. "They're not believed to be involved in the incident," Haug said.

The wounded were dispersed among hospitals in central Texas, Cone said.

Lisa Pfund of Random Lake, Wis., says she learned that her daughter Amber Bahr, 19, was shot in the stomach but was in stable condition. "We know nothing, just that she was shot in the belly," Pfund told the Associated Press. She couldn't provide more details and had spoken only with emergency personnel.

A Fort Hood spokesman said he could not immediately confirm any identities of the injured.

Tribune Newspapers reported that the base was locked down after the shootings, and people who live there were told to lock their doors and windows.

Families, so used to being separated during long deployments, were separated again in a situation that, to many, seemed surreal.

"My friend's husband called her from Iraq and said, 'Isn't it sad that I am safer over here in Iraq than you are at home?' " said Jessica Sullens, 28, who had spent hours in a nearby Wal-Mart parking lot, where she had dashed on a midday errand. Her own husband, Cpl. Thomas Sullens, and their 1- and 2-year-old daughters were in lockdown on the base - he with his motor pool, the children with a neighbor. "This is unreal to me," Sullens said.

Fort Hood, which sprawls across 339 square miles of central Texas hill country, is the world's largest military installation. It supports two full armored divisions - the 1st Cavalry Division and the 4th Infantry Division - and is home to more than 70,000 soldiers, civilian workers and family members. It is the largest single employer in Texas.

Base personnel have accounted for more suicides than any other Army post since the U.S. invasion of Iraq in 2003, with 75 tallied through July of this year, possibly from long and repeated deployments to Iraq and Afghanistan. Nine of those suicides occurred in 2009, counting two in overseas war zones.