A soldier opened fire on fellow service members at the Fort Hood military base, killing three people and wounding 16 before committing suicide at the same post where more than a dozen people were slain in a 2009 attack, authorities said.
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.
There was no indication the Wednesday attack was related to terrorism, Milley said.
A Texas congressman said the shooting happened at a medical centre. 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. Dr. 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 U.S. 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.
On Wednesday evening, a fatigue-clad soldier and a military police officer stood about a quarter-mile from the main gate waving away traffic. Other lanes were blocked by a police car and van.
U.S. President Barack Obama vowed that investigators would get to the bottom of the shooting.
In a hastily arranged statement in Chicago, 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 valour. 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.''
The November 2009 attack happened inside a crowded building where soldiers were waiting to get vaccines and routine paperwork after recently returning from deployments or preparing to go to Afghanistan and Iraq.
Army psychiatrist Nidal Hasan was convicted and sentenced to death last year in that mass shooting. He said he acted to protect Islamic insurgents abroad from American aggression.
In September, a former Navy man opened fire at the Washington Navy Yard, leaving 13 people dead, including the gunman. After that shooting, Defence Secretary Chuck Hagel ordered the Pentagon to review security at all U.S. defence installations worldwide and examine the granting of security clearances that allow access to them.
Asked Wednesday about security improvements in the wake of other shootings at U.S. military bases, Hagel said, ``Obviously when we have these kinds of tragedies on our bases, something's not working.''