Twelve people have been killed and 31 wounded in a shooting spree at a Texas military base, Ft Hood, in a murderous rampage that officials believe was carried out by an Army officer.

Ten of the other dead also were soldiers, while the remaining one was a civilian police officer who was working as a contractor on the base, Cone said.

The slain gunman was identified as Maj. Malik Nadal Hasan, a convert to Islam, according to a law enforcement source. The source believes he is 39 or 40 years old.

Two of the injured were in "very serious" condition, Fort Hood spokesman Christopher Hogue said.

"All the casualties took place at the initial incident, that took place at 13:30 [1:30 p.m. CT], at the soldier readiness facility," Cone said.  "The gunman opened fire and essentially due to the quick respond of the police forces was killed." 

A congressional aide said he was on the post to attend a graduation service when he saw a soldier with blood on his uniform near the building where the service was being held, the Austin American-Statesman reported.

Greg Schannep, an aide to U.S. Rep. John Carter, said the soldier ran past him and said a man was shooting. He said the soldier appeared to be injured in a shoulder, the American-Statesman reported.

FBI agents are headed to the scene to assist, said Erik Vasys, spokesman for the FBI office in San Antonio. He had no other details.

The gunman's suspected accomplices were taken into custody in an adjacent facility known as the old SportsDome Complex.