12 people were killed and around 31 were injured Thursday when a lone gunner opened fire at civilians and army personnel at Fort Hood, the largest Army base in the Unites States.

The shooter is believed to be a psychiatrist named Nidal Malik Hasan (Malik Nadal Hasan in some reports). He was a psychiatrist at Darnall Army Medical Center on the base.

A military source was quoted by the Air Force Times as saying Hasan had recently been reassigned to Fort Hood from Walter Reed Army Medical Center in Washington. In 2009, sources tell ABC News, he completed a fellowship in Disaster and Preventative Psychiatry at the Center for Traumatic Stress there.

The shooter opened fire on fellow army personnel and civilians in a processing center for soldiers being deployed to Iraq and Afghanistan. The shooter used two handguns and was quickly killed by military personnel and police stationed at the army base. Two other suspects were arrested after the gun battle.

Update: The shooter, Nidal Malik Hasan, is still alive and is now in stable condition. The gunman, first said to have been killed, was wounded but alive and in stable condition under military guard, said Lt. Gen. Bob Cone at Fort Hood. “I would say his death is not imminent,” Cone said. Col. Ben Danner said the suspect was shot four times and was in critical condition.

(via ABC News, update from HuffPo | photo credit: ABC News)