FORT HOOD, Texas — A military mental health doctor facing deployment overseas opened fire at the Fort Hood Army post on Thursday, setting off on a rampage that killed 12 people and left 31 wounded, Army officials said.

Authorities said immediately after the shootings that they had killed the suspected shooter, but later in the evening they recanted and said that he was alive and in stable condition at a hospital, watched by a guard.

“His death is not imminent,” said Lt. Gen. Bob Cone at Fort Hood. He offered little explanation for the mistake, other than to say there was confusion at the hospital.

A law enforcement official identified the shooting suspect as Army Maj. Nidal Malik Hasan. The official spoke on condition of anonymity because he was not authorized to discuss the case publicly.

The violence was believed to be the worst mass shooting in history at a U.S. military base.

The shooting began around 1:30 p.m., when shots were fired at the base’s Soldier Readiness Center, where soldiers who are about to be deployed or who are returning undergo medical screening, Cone said.

It was unclear what the motive was, though it appeared he was upset about a scheduled deployment. U.S. Sen. Kay Bailey Hutchison said the Army major was about to deploy overseas, though it was unclear if he was headed to Iraq or Afghanistan and when he was scheduled to leave. Hutchison said she was told about the upcoming deployment by generals based at Fort Hood.