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 13 people and left 30 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.”

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.

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.

Cone said initially three people were held, and all have been interviewed. Authorities believe, however, that there was a single shooter.

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.”

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.

The Marine Corps Air Ground Combat Center in Twentynine Palms increased security precautions at gates leading into the base after the Ft. Hood incident, Public Affairs Officer Jennie Haskamp said Thursday afternoon.

The base began checking the identification of every person coming aboard the base and conducting random vehicle searches, Haskamp said.

The La Quinta LifeStream donor center shipped 10 units of O-Negative blood to Ft. Hood on Thursday afternoon, said Rosalie Ornelas-Erlandson, LifeStream public relations specialist.

Brian Indrelunas and Aldrich M. Tan of The Desert Sun contributed to this report.