Army psychiatrist Major Nidal Hasan was charged Thursday with 13 counts of premeditated murder in last week's Fort Hood mass shootings.

Hasan, 39, is suspected of killing 13 of his comrades Nov. 5 when he opened fire at a soldier processing center at the Army base in Killeen, Texas. He was charged in the military's legal system, making him eligible for the death penalty.

SLIDESHOW: Fort Hood Victims

The Army Criminal Investigation Command formally announced the charges against Hasan at Fort Hood Wednesday afternoon.

"We are aggressively following every possible lead," said Chris Grey with the Army's Criminal Investigation Command, adding that additional charges are possible.

SLIDESHOW: Fort Hood Massacre

Hasan will be tried in the military's court-martial system. Prosecutors will likely seek the death penalty, the maximum sentence he faces. The minimum is life in prison.

Executions of military defendants are very rare.

More on Hasan's chances at the death penalty.

President Barack Obama ordered a review of all intelligence related to Hasan, and whether the information was properly shared and acted upon within government agencies.

"We have a duty and obligation to protect the constitutional rights of everyone involved," Grey said.

Officials told The Associated Press before the news conference that it had not been decided whether to charge Hasan with a 14th count of murder related to the death of the unborn child of a pregnant shooting victim. The officials spoke on condition of anonymity because they were not authorized to speak about the case publicly.

John Galligan, Hasan's civilian attorney, said his military co-counsel told him that charges were being read to Hasan in the hospital without his lawyers present.

"I don't like it. I feel like I'm being left out of the loop," Galligan said. "I guess it's 13 charges, but I don't like to have to guess in this situation."

The American-born military psychiatrist survived the rampage and is being guarded at a hospital in San Antonio. He has been talking to investigators.

Hasan is accused of firing about 100 rounds at unsuspecting fellow soldiers filling out deployment paperwork before civilian police officers shot him and stopped the attack.

The massacre killed 13 people and left about 30 others wounded. It has been classified as the worst mass shooting on an American military base.

Hasan's motives are being investigated.

Reports have surfaced about possible communications he may have had with terrorist groups, fear of his imminent deployment to the war in Afghanistan and potential psychiatric problems, but authorities working on the case have declined to confirm or comment on motive theories.

Months before last week's shootings, doctors and staff overseeing Hasan's training reported viewing him at times as belligerent, defensive and argumentative in his frequent discussions of his Muslim faith, according to a military official familiar with several group discussions about Hasan.

The official asked that his name be withheld because he was not authorized to speak publicly about the meetings.

Hasan was characterized as a mediocre student and lazy worker, a matter of concern among the doctors and staff at Walter Reed Army Medical Center and the Uniformed Services University of the Health Sciences, a military medical school in Bethesda, Md., the official said.

The concerns about Hasan's performance and religious views were shared with other military officials considering his assignment after he finished his medical training, and the consensus was to send the psychiatrist to Fort Hood in Texas, the official said.

One of the largest military installations, it was considered the best assignment for Hasan because other doctors could handle the workload if he continued to perform poorly and his superiors could document any continued behavior problems, the official said.

Hasan repeatedly referred to his strong religious views in discussions with classmates, his superiors and even in his research work, the official said. His behavior, while at times perceived as intense and combative, was not unlike the zeal of others with strong religious views, the source said.

Citing the investigation and the Privacy Act, the Army and the Uniformed Services University of the Health Sciences have released only minimal details of Hasan's career. He entered the Army in 1997 as a 2nd lieutenant and started the medical school program, according to a service spokesman in Washington.

But school records from Barstow Community College in Barstow, Calif., where Hasan was a student from 1989 to 1990, show his military service began much earlier. Maureen Stokes, a spokeswoman for the college, said the records indicate he was a private first class with an infantry unit at Fort Irwin, Calif. Hasan received 10 credits for his military experience, she said.

John Wagstaffe, a Fort Irwin spokesman, said that based upon the school records it would appear that Hasan was stationed at Fort Irwin. But he said base officials have not been able to locate the military records to verify that.

The Pentagon has found no evidence that Hasan formally sought release from the Army as a conscientious objector or for any other reason, two senior military officials told The Associated Press.

Family members have said he wanted to get out of the Army and had sought legal advice, suggesting that Hasan's anxiety as a Muslim over his pending deployment overseas might have been a factor in the deadly rampage.

Hasan had complained privately to colleagues that he was harassed for his religion and that he wanted to get out of the Army.

But there is no record of Hasan filing a complaint with his chain of command regarding any harassment he may have suffered for being Muslim or any record of him formally seeking release from the military, the officials told the AP.

The Associated Press contributed to this report.