Investigators were still trying to piecing together how and why an Army psychiatrist facing deployment allegedly gunned down his comrades in one of the worst mass shootings ever on an American military base.

“This was an individual who took it upon himself to attack and murder his colleagues, people who were on the base with him,” Homeland Security Secretary Janet Napolitano told Sky News from Brussels, Belgium. “That investigation is under way by law enforcement authorities, and let’s let that be the No. 1 priority in terms of ascertaining what motivations he had.”

Officials at the post hospital where Hasan worked said they weren’t aware of any problems with his job performance.

One of Hasan’s bosses praised his work ethic and said he provided excellent care for his patients.

“Up to this point I would consider him an asset,” said Col. Kimberly Kesling, deputy commander of clinical services at Darnall Army Medical Center.

She described Hasan as “a quiet man who wouldn’t seek the limelight.”

An imam from a mosque Hasan regularly attended said Hasan, a lifelong Muslim, was a committed soldier, gave no sign of extremist beliefs and regularly wore his uniform at prayers.

Soldiers who witnessed the rampage reported that the gunman shouted “Allahu Akbar!” — an Arabic phrase for “God is great!” — before opening fire, said Lt. Gen. Robert Cone, the base commander. He said officials had not yet confirmed that Hasan made the comment before the shooting spree.

Hasan’s family said in a statement Friday that his alleged actions were “despicable and deplorable” and don’t reflect how the family was raised.

Villa, who recently moved next door to Hasan, said she had never spoken to him before he came over to her apartment.

She said Hasan gave her frozen broccoli, spinach, T-shirts and shelves on Wednesday, then returned Thursday morning and gave her his air mattress, several briefcases and a desk lamp. He then offered her $60 to clean his apartment Friday morning, after he was supposed to leave.

The motive for the shooting wasn’t clear, but someone who used to work with Hasan said he had expressed some anger about the wars in Iraq and Afghanistan.

Retired Col. Terry Lee told Fox News said Hasan had hoped President Barack Obama would pull troops out of Afghanistan and Iraq and got into frequent arguments with others in the military who supported the wars.

But another neighbor said Hasan appeared to be OK with his pending deployment, which he said was supposed to be to Afghanistan.

“I asked him how he felt about going over there, with their religion and everything, and he said, ‘It’s going to be interesting,’” said Edgar Booker, a 58-year-old retired soldier who now works in a cafeteria on the post.

Col. Steve Braverman, the Fort Hood hospital commander, said early Friday that Hasan was on deployment orders to Afghanistan. A military official later told The Associated Press that Hasan was to be deployed to Iraq. It was not immediately possible to verify the discrepancy.

The military official, who did not have authorization to discuss the matter publicly and spoke to the AP on condition of anonymity, said Hasan had indicated he didn’t want to go to Iraq but was willing to serve in Afghanistan.

Cone said authorities have not yet been able to talk to Hasan, but interviews with witnesses went through the night.

As some of the wounded began to recover, tales of heroic action during the shooting spree emerged.

Base officials lauded an officer, Kimberly Munley, who shot the gunman and was wounded herself.

“She happened to encounter the gunman. In an exchange of gunfire, she was wounded but managed to wound him four times,” Cone said. “It was an amazing and aggressive performance by this police officer.”

Cone said some 300 soldiers had been lined up to get vaccinations and have their eyes tested at a Soldier Readiness Center when the shots rang out. He said one soldier who had been shot told him, “I made the mistake of moving and I was shot again.”

Sgt. Andrew Hagerman said before the first ambulance even arrived, soldiers were tearing off their clothes to help the wounded.

“You had people without tops on. You had people ripping their pant legs off,” said Hagerman, a military policeman from Lewisville, Texas.

Hagerman said he saw Hasan laying on the ground receiving medical assistance for a gunshot wound as responders tried to get his handcuffs off to better treat him.

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

Cone acknowledged that it was “counterintuitive” that a single shooter could hit so many people, but he said the massacre occurred in “close quarters.

“With ricochet fire, he was able to injure that number of people,” Cone said. He said authorities were investigating whether Hasan’s weapons were properly registered with the military.

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

Friday was designated a day of mourning at Fort Hood. There also will be a ceremony at the air base to honor the dead.

For six years before reporting for duty at the Texas post in July, Hasan 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. The Army major 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.

Faizul Khan, a former imam at a mosque Hasan attended in Silver Spring, Md., said “I got the impression that he was a committed soldier.” He said Hasan attended prayers regularly at the mosque in Silver Spring, Md., and was a lifelong Muslim. He spoke often with Hasan about Hasan’s desire for a wife.

In an interview with The Washington Post, Hasan’s aunt, Noel Hasan of Falls Church, Va., said he had been harassed about being a Muslim in the years after the Sept. 11, 2001, terror attacks, and he wanted out of the Army.

“Some people can take it and some people cannot,” she said. “He had listened to all of that and he wanted out of the military.”

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.

FBI agents who searched Hasan’s apartment early Friday seized his computer, a law enforcement official said. It was not immediately known if they found anything suspicious on his computer files.

A military official said investigators were sifting through materials Hasan carried with him during the shooting and evidence left in his vehicle, which was found parked at the base.

The officials spoke on condition of anonymity because of the ongoing investigation

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Baker reported from Killeen, Texas. Associated Press Writers Lara Jakes, Devlin Barrett, Brett J. Blackledge and Eileen Sullivan in Washington, April Castro in Killeen and Matt Curry in Dallas contributed to this report.

Update: At his Thursday night press conference, Cone attributed the mistaken report about Hasan being dead to “confusion at the hospital.” The briefing had been delayed for some time while the general and others worked to get all the facts about Hasan’s condition.

The alleged shooter has been in custody “throughout the hospitalization process,” Cone said. He is not currently speaking with investigators, though according to Cone, “his death is not imminent.”

Two other people were initially detained, but were released after being interviewed. The two were involved in an altercation, Bruce Ziesdorf, a public affairs officer at Fort Hood, told Salon, and were “apprehended because, in the emotion of the situation, they were acting abnormally.”

The number of people reported dead stands at 12, even with Hasan alive.

There’s been some confusion and some conflicting reports about details of the incident because of the chaos that naturally follows anything like this, but Hutchison said she’s been told there were two shooters in this case. Military police wounded one of the shooters, Hutchison said, while the other was not yet apprehended. Also appearing on MSNBC, Chris Haug, a Ft. Hood spokesman, said one shooter has been apprehended and that officials are “investigating the scene for possible other shooters.”

It is far too early to guess at the motive behind the shooting. It is not even clear at this point that the person or persons responsible are soldiers. The trauma of going to war, however — or of going back to war for a second or third time — is considered an emotional flashpoint for troops and a possible trigger for violence, including murder and suicide.

Update: At a press conference, Lt. Gen. Bob Cone told reporters that the shooter was among those killed in the incident as authorities responded to the violence, but did not specify if the shooter is counted among the 12 reported dead. Cone said two more soldiers had also been apprehended as suspects in the shooting, but he provided no new details about the circumstances surrounding those suspects. He did say that the shooter used two weapons, both handguns.

“It is a terrible tragedy,” Cone said. “It is stunning.”

Contrary to swirling reports that additional shootings had occurred at other places on post, Cone said all of the violence occurred at the processing center at 1:30 p.m. local time.

President Obama, too, has spoken about the shooting, making brief remarks at an unrelated event Thursday afternoon. He noted that details remain sketchy, but said, “What we do know is that a number of American soldiers have been killed and even more have been wounded in a horrific outburst of violence.”

Obama added, “My immediate thoughts and prayers are with the wounded and with the families of the fallen, and with those who live and serve at Fort Hood. These are men and women who have made the selfless and courageous decision to risk and at times give their lives to protect the rest of us on a daily basis. It’s difficult enough when we lose these brave Americans in battles overseas. It is horrifying that they should come under fire at an Army base on American soil.”

Update 2: Multiple news outlets are now identifying the shooter as Army Maj. Malik Nadal Hasan, who’s believed to be in his late 30′s.

Update 3: The Associated Press is reporting that Hasan had been a psychiatrist at Walter Reed Army Medical Center, and that he was transfered to Fort Hood this summer. The AP cites unnamed “military officials” as saying Hasan had received a poor performance evaluation at Walter Reed.

Update 4: At a second press conference Thursday night, Cone said that Nadal is not dead, but was shot. As of this update, the alleged shooter is in custody and hospitalized in stable condition. See this post for more.

Banks says the second shooting took place at a theater on the sprawling base.

An Army spokesman said the base was locked down after the shootings.

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, the sprawling base is located halfway between Austin and Waco.

At the Soldier Readiness Center, soldiers who are about to be deployed or who are returning undergo medical screening — on average about 300-400 screened a day, Sgt. Rebekah Lampam, a Fort Hood spokeswoman, said.

Lampam said a graduation ceremony for soldiers who finished college courses while deployed was going on in the auditorium at the time of the shooting.

The White House said President Barack Obama was notified of the shootings.

The base is home to nine schools — seven elementary schools and two middle schools — and all were on lockdown, said Killeen school spokesman Todd Martin.

Texas Department of Public Safety spokeswoman Tela Mange said Texas Rangers and state troopers were en route to Fort Hood to help seal the perimeter of the 108,000 acre base.

Fort Hood officially opened on Sept. 18, 1942, and was named in honor of Gen. John Bell Hood. It has been continuously used for armored training and is charged with maintaining readiness for combat missions.

AP National Security Writer Anne Gearan and Associated Press writers Lolita C. Baldor and Devlin Barrett in Washington, D.C., and Associated Press Writer Linda Stewart Ball in Dallas contributed to this report.