The suspected shooter in the Fort Hood, Texas rampage has been charged with 13 counts of premeditated murder and will be tried in a military court, the US Army announced on Thursday.
Major Nidal Malik Hasan is accused of opening fire with two handguns on November 5 at a Fort Hood facility for preparing troops for overseas deployment. He could face the death penalty.
Hasan, a 39 year-old Army psychiatrist, is believed to have carried out the worst shooting ever at a US military installation. The shooting spree left 12 soldiers and a civilian employee at the base dead. Forty-three others were wounded.
Hasan could face additional charges as the investigation continues, said Christopher Grey, a spokesman for the Army's criminal investigation division.
Authorities were still investigating the motive behind the attack. There have been reports that Hasan, a devout Muslim, was disgruntled over his pending deployment to the war in Afghanistan.
"We are doing everything possible and we are looking at every reason for this shooting," Grey said. "We are aggressively following every possible lead."
During his years in the military, Hasan had reportedly become increasingly vocal about his religious faith and opposition to the wars in Afghanistan and Iraq.
Hasan suffered gunshot wounds from two police officers who are credited with ending the rampage. Hasan remained hospitalised and was expected to recover. He has been informed of the charges and obtained legal representation, Grey said.
Meanwhile, US President Barack Obama on Friday ordered the military and other government agencies to gather any documents about Hasan prior to the attack to determine if there were signs he could turn violent, the White House said on Thursday.