FORT HOOD, Texas — An Army psychiatrist set to be shipped overseas opened fire at the Fort Hood Army post Thursday, authorites said, a rampage that killed 12 people and left 31 wounded in the worst mass shooting ever at a military base in the United States.

The gunman, first said to have been killed, was wounded but alive and in stable condition under military guard, said Lt. Gen. Bob Cone at Fort Hood. "I would say his death is not imminent," Cone said. Col. Ben Danner said the suspect was shot at least four times.

The man was identified as Maj. Nidal Malik Hasan, a 39-year-old, eight-year veteran from Virginia.

President Barack Obama called the shooting at the Soldier Readiness Center, where soldiers who are about to be deployed or who are returning undergo medical screening, "a horrific outburst of violence."

"It's difficult enough when we lose these brave Americans in battles overseas," the commander in chief said. "It is horrifying that they should come under fire at an Army base on American soil."

There was no official word on motive. Hasan had transferred to Fort Hood in July from Walter Reed Medical Center, where he received a poor performance evaluation, according to an official who spoke on condition of anonymity because he was not authorized to discuss the case publicly.

Sen. Kay Bailey Hutchison, R-Texas, said generals at Fort Hood told her that Hasan was about to deploy overseas. Retired Col. Terry Lee, who said he had worked with Hasan, told Fox News he was being sent to Afghanistan.

Lee said Hasan had hoped Obama would pull troops out of Afghanistan and Iraq and got into frequent arguments with others in the military who supported the wars.

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.

Video from the scene showed police patrolling the area with handguns and rifles, ducking behind buildings for cover. Sirens could be heard wailing while a woman's voice on a public-address system urged people to take cover.

"I was confused and just shocked," said Spc. Jerry Richard, 27, who works at the center but was not on duty during the shooting. "Overseas you are ready for it. But here you can't even defend yourself."

Soldiers at Fort Hood don't carry weapons unless they are doing training exercises.

The Rev. Greg Schannep was about to head into a graduation ceremony when a man in uniform approached him, warning him that someone had opened fire. Schannep heard three volleys of gunfire and saw people running.