FORT HOOD, Texas — Military officials were starting Friday to piece together what may have pushed an Army psychiatrist trained to help soldiers in distress to turn on his comrades in a shooting rampage that killed 13 people and wounded 28 in Texas.

The suspected shooter, Maj. Nidal Malik Hasan, was on a ventilator and unconscious in a hospital after being shot four times during the incident at the Army's sprawling Fort Hood, post officials said. In the early chaos after the shootings, authorities believed they had killed him, only to discover later that he had survived.

In Washington, a senior U.S. official said authorities at Fort Hood initially thought one of the victims who had been shot and killed was the shooter. The mistake resulted in a delay of several hours in identifying Hasan as the alleged assailant.

Authorities have not ruled out that Hasan was acting on behalf of some unidentified radical group, the official said. He would not say whether any evidence had come to light to support that theory.

The official spoke on condition of anonymity to discuss matters that were under investigation.

Officials are not ruling out the possibility that some of the casualties may have been victims of "friendly fire," that in the mayhem and confusion at the shooting scene some of the responding military officials may have shot some of the victims.

The gunfire broke out around 1:30 p.m. at the Soldier Readiness Center, where soldiers who are about to be deployed or who are returning undergo medical screening. Nearby, some soldiers were readying to head into a graduation ceremony for troops and families who had recently earned degrees.

Pastor Greg Schannep had just parked his car along the side of the theater and was about to head into the ceremony when a man in uniform approached him.

"Sir, they are opening fire over there!" the man told him. At first, he thought it was a training exercise; then heard three volleys and saw people running. As the man who warned him about the shots ran away, he could see the man's back was bloodied from a wound.

Schannep said police and medical and other emergency personnel were on the scene in an instant, telling people to get inside the theater. The post went into lockdown while a search began for a suspect and emergency workers began trying to treat the wounded. Some soldiers rushed to treat their injured colleagues by ripping their uniforms into makeshift bandages to treat their wounds.

Five members of the North Carolina National Guard's 130th Attack Reconnaissance Battalion, which is stationed in Morrisville, were at Fort Hood for final processing before deployment to Iraq, officials said. None was injured in the shooting spree, they said.

Fort Hood Lt. Gen. Bob Cone praised the soldiers for their quick reaction.

"God bless these soldiers," Cone said. "As horrible as this was it could have been worse."

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. Schools on the base went into lockdown, and family members trying to find out what was happening inside found cell phone lines jammed or busy.

"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."

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

The bodies of the victims would be taken to Dover Air Force Base in Delaware for autopsies and forensic tests, said a U.S. official who spoke on condition of anonymity to discuss matters that were under investigation.

There will also be a ceremony at the air base to honor the dead.

Jamie and Scotty Casteel stood outside the emergency room at the hospital in Temple waiting for news of their son-in-law Matthew Cooke, who was among the injured.

"He's been shot in the abdomen and that's all we know," Jamie Casteel told The Associated Press. She said Cook, from New York state, had been home from Iraq for about a year.

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 said. She couldn't provide more details and only spoke with emergency personnel.

Ashley Saucedo told WOOD-TV in Michigan that her husband was shot in the arm, but she couldn't discuss specifics. Saucedo said she and the couple's two children weren't permitted to leave their home at Fort Hood during the shootings.

The motive for the shooting wasn't clear, but Hasan was apparently set to deploy soon, and had expressed some anger about the wars in Iraq and Afghanistan. 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.

For six years before reporting for duty at Fort Hood, in July, the 39-year-old Army major worked at the Walter Reed Army Medical Center pursuing a career in psychiatry, as an intern, a resident and, last year, a fellow in disaster and preventive psychiatry. He received his medical degree from the military's Uniformed Services University of the Health Sciences in Bethesda, Md., in 2001.

But his record wasn't sterling. At Walter Reed, 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. And while he was an intern, Hasan had some "difficulties" that required counseling and extra supervision, said Dr. Thomas Grieger, who was the training director at the time.

At least six months ago, Hasan came to the attention of law enforcement officials because of Internet postings about suicide bombings and other threats, including posts that equated suicide bombers to soldiers who throw themselves on a grenade to save the lives of their comrades.

Investigators had not determined for certain whether Hasan was the author of the posting, and a formal investigation had not been opened before the shooting, said law enforcement officials who spoke on condition of anonymity because they are not authorized to discuss the case.

The FBI, local police and other agencies searched Hasan's apartment Thursday night after evacuating the complex in Killeen, said city spokeswoman Hilary Shine. She referred questions about what was found to the FBI. The FBI in Dallas referred questions to a spokesman who was not immediately available early Friday morning.

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Associated Press Writers Lara Jakes in Washington and Matt Curry in Dallas contributed to this report.