Maj. Nidal Malik Hasan has been charged with 13 counts of premeditated murder in connection with a shooting rampage at Fort Hood, Texas, on Nov. 5 that left 13 people dead. (Uniformed Services University of the Health Sciences/Associated Press)

The U.S. army psychiatrist accused of the mass shooting at Fort Hood has been charged with 13 counts of premeditated murder under the American military's legal system.

The announcement of formal charges against Maj. Nidal Malik Hasan comes on the same day U.S. President Barack Obama ordered a review of all intelligence related to Hasan and whether it was properly shared within individual government agencies.

Hasan, 39, is the only suspect in the Nov. 5 shooting rampage at the huge Texas army base in which an individual opened fire in a room crowded with hundreds of soldiers, killing 13 and wounding 29.

Premeditated murder, the most serious offence in a military court, makes Hasan eligible for the death penalty if convicted. The U.S. military justice system has not executed anyone since 1961, though five servicemen are currently on death row.

Criminal investigation command spokesman Christopher Grey said at a news conference Thursday that additional charges may be laid as the investigation continues.

Military officials have suggested investigators are considering the addition of a 14th murder charge for the unborn child of one of the shooting victims.

Military criminal justice consultant Gregory D. Lee told CBC News that Hasan could face charges unique to the military courts, such as conduct unbecoming an officer, or even charges of sedition because he attacked fellow soldiers while a member of the military.

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."

Warning signs before shooting

The announcement of the White House review comes as members of the U.S. Congress press for a full investigation on why Hasan was not detected and stopped when he entered the base. The Senate's Homeland Security committee announced it is opening its own investigation this week.

The first results of the White House review are due by Nov. 30.

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, a military official told The Associated Press on the condition of anonymity.

The Washington Post reported Tuesday that Hasan warned his medical colleagues 18 months ago that to "decrease adverse events" the U.S. military should allow Muslim soldiers to be released as conscientious objectors instead of fighting in wars against other Muslims.

Hasan was reportedly set to begin a tour in either Iraq or Afghanistan, and was unhappy with the move, according to his family.

Military officials are also reviewing communications between Hasan and a radical imam overseas that prompted an investigation by the Federal Bureau of Investigation. Hasan reportedly communicated 10 to 20 times with Anwar al-Awlaki, a former imam from Virginia who had alleged links to three of the Sept. 11, 2001, hijackers.

Investigators said the content of Hasan's emails was "consistent with the subject matter of his research" as someone who worked with post-traumatic stress disorder cases as a result of combat in Iraq and Afghanistan.

With files from The Associated Press