FORT HOOD, Texas — An Army psychiatrist who had recently completed training in treating traumatic stress disorder opened fire Thursday on a group of soldiers preparing to deploy to Iraq, killing at least 12. Another 31 were wounded or injured.

Post commander Lt. Gen. Robert Cone told reporters late Thursday that the shooter, identified as Maj. Nidal Malik Hasan, was in custody and that he was expected to live, despite having been shot multiple times. Earlier, Cone had reported that Hasan had been killed.

Cone also said late Thursday that Hasan hadn't spoken to investigators.

It was unclear whether the gunman acted alone. Cone said three soldiers who had been taken into custody as possible accomplices had been released. Cone said that while there was nothing suggesting terrorism, "I couldn't rule that out."

Cone said two of the 31 wounded were civilians. The identities of the dead and wounded weren't released.

"It's a terrible tragedy. It's stunning," Cone said.

Fort Hood, adjacent to the city of Killeen and about 150 miles south of Dallas, is the largest active-duty armored post in the U.S., with about 52,000 troops from the 1st Cavalry Division, 3rd Armored Cavalry Regiment, 4th Infantry Division and other units stationed there. More than 15,000 of its soldiers are currently deployed in Iraq.

Hasan used two handguns during the attack, Cone said, which began about 1:30 p.m. at a personnel and medical processing center on the post.

After Hasan began firing, he was shot by a police officer, who also was wounded, Core said. Cone noted that soldiers on the post do not ordinarily carry weapons.

"As a matter of practice, we do not carry weapons," he said. "This is our home."

Cone said that as the shooting unfolded, bystanders quickly sealed off an auditorium about 50 yards away where a graduation ceremony was being held for 138 soldiers who had completed correspondence courses during their deployment. About 600 friends and family members were in the audience, Cone said.

Greg Schannep, the regional director for U.S. Rep. John Carter, R-Texas, whose district includes Fort Hood, was representing the congressman at the graduation, said John Stone, a spokesman for Carter.

Schannep was at the ceremony when a soldier who had been shot in the back came running toward him and alerted him of the shooting, Stone said. The soldier told Schannep not to go in the direction of the shooter, he said.

Cone said all the victims were gunned down "in the same area" inside what he called a Soldier Readiness Processing Center.

As the shootings ended, scores of emergency vehicles rushed to the scene, which is in the center of the post, and dozens of ambulances carried the shooting victims to hospitals in the region.

Both of the handguns used by Hasan were recovered at the scene. Investigators said the major's computers, cell phones and papers would be examined, his past investigated and his friends, relatives and military acquaintances would be interviewed in an effort to develop a profile of him and try to learn what had motivated his deadly outburst.

The weapons he used were described as "civilian" handguns. Security experts said the fact that two handguns were used suggested premeditation, as opposed to a spontaneous act.

Rifles and assault weapons are conspicuous and not ordinarily seen on the streets of a military post, and medical personnel would have no reason to carry any weapon, they said. Moreover, security experts noted, it took a lot of ammunition to shoot 43 people, another indication of premeditation.

The post went into lockdown shortly after the shootings. Gates were closed and barriers put up at all entrance and exit checkpoints, and the military police turned away all but essential personnel. Schools on the post were closed, playgrounds were deserted and sidewalks were empty. Sirens wailed across the post through the afternoon, a warning to military personnel and their families to remain indoors.

Distraught Fort Hood family members, including parents rushing to pick up their children from day care, gathered at the main gate after the post was locked down.

"When I first heard, I was in tears," said Cynthia Wood, whose son Conner was in day care on the post. "It's very disheartening not being able to get your child."

As she was talking to reporters, her husband, Army Spc. Joshua Wood, was sending her a frantic text message from Iraq asking for more details.

Monica Cain brushed back tears as she explained that she was unable to reach her husband, whom she had taken to Fort Hood earlier in the day for medical treatment of a head injury he suffered in combat. Sgt. Barren Cain had told her he planned to call at 1 p.m. to summon her to pick him up. But by midafternoon, after news of the shooting spread across the region, she hadn't heard from him and was unable to reach him by cell phone.

"I don't know what's going on," she said. "I'm very scared."

She said the medical center wasn't far from the reported scene of the shooting.

Jutte Kobel said she was at an office job off-base at 2 p.m. when she got a mysterious call from her husband, Staff Sgt. Edward Kobel, at the post: "Don't worry, I'm fine," he said, and hung up.

She was bewildered until he called back 10 minutes later and filled her in. Then she turned on the television.

The Kobels live on post but had talked about moving because, she said, she worried that it wasn't safe. "Not really," she said. "On post, especially this one — the largest in the world — it might be the first place there'd be an attack."

A public affairs officer at the post said that sirens sounded on the post to alert all personnel to remain where they were. A scrolling notice on the post's website declared: "Organizations/units are instructed to execute a 100 percent accountability of all personnel. This is not a drill. It is an emergency situation."

Officials at the Pentagon said the wounded were taken to Darnall Army Medical Center for treatment.

Meanwhile, 30 minutes away in Temple, Texas, hundreds of people responded to calls to give blood and showed up at the main hospital for Scott & White Healthcare System to donate.

They included Travis Huckabee and his wife, Lee Ann Huckabee, who had been standing in line for two hours by 7 p.m. They had heard the plea on radio.

"If we were ever in a situation where our children needed blood, this is our chance to pay forward that need," said Lee Ann Huckabee, the mother of two preschool girls.

Standing behind them was Elizabeth Flores, 19, of Killeen, Texas, the daughter of an active-duty soldier. She was already at hospital visiting her uncle, a patient, when she heard over the intercom the plea for donors about 5 p.m.

"If my blood is a match to anyone, then I feel that I may help save someone's life," she said.