Maj. Nidal Malik Hasan, an Army psychiatrist who allegedly opened fire on dozens of people at Fort Hood Army Post, cleaned out his apartment the morning of the shootings, his neighbors said Friday.

Several residents at the apartment complex in Killeen, Texas, said he also gave them copies of the Quran hours before Thursday's massacre that killed 13 people and wounded 38.

FBI agents helping investigate the shootings at the largest U.S. military base searched Hasan's apartment while investigators pieced through the gruesome crime scene, Fort Hood's military processing center, where soldiers report before they go to war.

Hasan, who worked at a hospital on the base, is accused of using two handguns to fatally shoot 12 soldiers and one Army civilian employee, military officials said. He was transported by air Friday afternoon to Brooke Army Medical Center in San Antonio, and was in critical condition but stable, a spokesman said.

Two law enforcement sources told CNN that one of the weapons used was an FN 5.7-millimeter pistol, a semiautomatic purchased legally at Guns Galore, a Killeen gun shop. Details on the other gun, identified only as a type of revolver, were not immediately available.

Fort Hood's commanding general said witnesses have reported that the gunman yelled "Allahu Akbar," Arabic for "God is great," during the rampage. However, Lt. Gen. Robert Cone said investigators had not confirmed that.

The police officer who shot Hasan, ending the massacre, was among the wounded. Officer Kimberly Munley of the Fort Hood Police Department "just happened very fortunately to be very close to the incident scene," Cone told CNN's "American Morning," calling it "a pretty amazing and aggressive performance by this police officer."

Most of the wounded required hospitalization. About half required surgery, and all were in stable condition Friday morning, said Col. Steven Braverman, hospital commander at Fort Hood.

Officials were looking into whether some soldiers may have been shot accidentally by others trying to shoot the gunman.

President Obama, in remarks Friday morning, cautioned against "jumping to conclusions" about what had triggered "one of the worst mass shootings ever to take place on an American military base."

He ordered that flags at the White House and other federal buildings be flown at half staff until Veterans Day, on Wednesday of next week. "This is a modest tribute to those who lost their lives, even as many were preparing to risk their lives for their country," the president said. "It's also a recognition of the men and women who put their lives on the line every day to protect our safety and uphold our values. We honor their service, we stand in awe of their sacrifice, and we pray for the safety of those who fight and for the families of those who have fallen."

Obama said he met with FBI Director Robert Mueller and representatives of other relevant agencies to discuss the investigation. And he promised that his administration will provide updates.

Hasan's next door neighbor, Patricia Villa, said he gave her his furniture and paid her $60 to clean his apartment hours before the shooting rampage.

"He told me he was leaving for Iraq or somewhere," Villa said. "I didn't think much of it."

Miguel Rivera, another resident of the apartment complex, said he saw Hasan throwing items away the morning of the shooting.

FBI agents searched an outdoor trash bin and removed a second bin from the apartment complex.

Hasan's next-door neighbor, Willie Bell, said Hasan had helped him set up his laptop and regularly tapped into Bell's wireless service. Bell, a maintenance man who didn't show up for work Thursday, said he was interviewed by the FBI for four hours that day and the laptop was seized.

He said he received two calls from Hasan early Thursday, one at 2:37 a.m. asking Bell to turn on the wireless service, and again at 5 a.m. to say he was moving.

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