The shooter, who served in Iraq in 2011, had been undergoing an assessment to determine whether he had post-traumatic stress disorder, according to Lt. Gen. Mark A. Milley, the senior officer on the base.

A Texas congressman said the shooting happened at a medical center. Rep. Michael McCaul, chairman of the House Homeland Security Committee, also identified the suspect as Ivan Lopez. But additional details about the gunman were not immediately available.

The injured were taken to Darnall Army Community Hospital at Fort Hood and other local hospitals. Glen Couchman, chief medical officer at Scott and White Hospital in Temple, said the first four people admitted there had gunshots to chest, abdomen, neck and extremities and that their conditions range from stable to "quite critical."

The 2009 assault on Fort Hood was the deadliest attack on a domestic military installation in US history. Thirteen people were killed and more than 30 wounded.

After the shooting began, the Army's official Twitter feed said the post had been locked down. Hours later, all-clear sirens sounded.

In a hastily arranged statement in Chicago, President Barack Obama said he was following the situation closely. He said the shooting brought back painful memories of the 2009 attack.

Obama reflected on the sacrifices that troops stationed at Fort Hood have made — including enduring multiple tours to Iraq and Afghanistan.

"They serve with valor. They serve with distinction, and when they're at their home base, they need to feel safe," Obama said. "We don't yet know what happened tonight, but obviously that sense of safety has been broken once again."

NTJ/NJF