FORT HOOD, Texas – An Army psychiatrist set to be shipped overseas opened fire at the Fort Hood Army post Thursday, authorities 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 in a hospital under military guard, said Lt. Gen. Bob Cone at Fort Hood. He was shot four times, and was on a ventilator and unconscious, according to military officials. "I would say his death is not imminent," Cone said.

The man was identified as Maj. Nidal Malik Hasan, a 39-year-old 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."

Soldiers rushed to treat their injured colleagues by ripping their uniforms into makeshift bandages. Officials have not ruled out the possibility that some casualties may have been victims of "friendly fire," shot by authorities amid the mayhem and confusion at the scene, said a senior U.S. official who spoke on condition of anonymity to discuss matters that were under investigation.

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.

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

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.

"There was a burst of shots and more bursts of shots and people running everywhere," said Schannep, who works for local Congressman John Carter.

The uniformed man who had warned him ran to the theater. Schannep said he could see the man's back was bloodied from a wound. The man survived, was treated and will be fine, Schannep said.

Cone said initially three people were held, and all have been interviewed. Authorities believe, however, that there was a single shooter. In Washington, the senior U.S. official said authorities at Fort Hood initially thought one of the slain victims was the shooter, a mistake that resulted in a delay of several hours in identifying Hasan as the suspect.

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 wounded were dispersed among hospitals in central Texas, Cone said. Their identities, and the identities of the dead, were not immediately released.

Amber Bahr, 19, was shot in the stomach but was in stable condition, said her mother, Lisa Pfund of Random Lake, Wis.

"We know nothing, just that she was shot in the belly," Pfund told The Associated Press. She couldn't provide more details and only spoke with emergency personnel.

Hasan, whose family said he was born in suburban Washington, is 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, Md., in 2001 and was at Walter Reed for six years for his internship, residency and a fellowship.

"We are shocked and saddened by the terrible events at Fort Hood today," his cousin, Nadar Hasan, said in a statement issued on behalf of their family. "We send the families of the victims our most heartfelt sympathies."

The attack happened just down the road from one of the worst mass shootings in U.S. history. On Oct. 16, 1991, George Hennard smashed his pickup truck through a Luby's Cafeteria window in Killeen, Texas, and fired on the lunchtime crowd with a high-powered pistol, killing 22 people and wounding at least 20 others.

No other shooting at a military base in the U.S. has been anywhere near as deadly as Thursday's. In 1993, a gunman at Fort Knox shot five civilian co-workers, killing three, and then fatally shot himself.

Around the country, some bases stepped up security precautions, but no others were locked down.

Covering 339 square miles, Fort Hood is the largest active duty armored post in the United States. Home to about 52,000 troops as of earlier this year, it is located halfway between Austin and Waco.