A soldier has killed three fellow U.S. military personnel at the Foot Hood army base in central Texas in an incident that also left 16 others wounded.
The base's commander, Lieutenant General Mark Milley, said late Wednesday the shooter then killed himself with his semi-automatic pistol as he was approached by a military police officer.
Milley said the motive was not known, but that authorities did not believe it was related to terrorism. Federal, military and local investigators are working to determine exactly what happened.
Milley said the shooter spent four months in combat in Iraq in 2011. He was undergoing treatment for depression and anxiety, and was in the process of undergoing the "lengthy process" of being tested for post-traumatic stress disorder.
President Barack Obama, speaking from Chicago, offered condolences, saying "we're heartbroken that this might have happened again." He referred to the wounded and their families as those "who have sacrificed so much for freedom," and said the situation at the base is fluid.
Shortly after the first reports of gunfire, television footage showed helicopters overhead as military police searched the base. Nearby civilian police agencies searched vehicles and helped seal the perimeter of the base, located south of the city of Waco.
The base went on lockdown for several hours, as military police and other law enforcement combed the heavily guarded facility.
Fort Hood was the scene of a mass shooting spree in 2009. Thirteen people were killed and more than 30 others wounded when an Army psychiatrist opened fire on personnel.
After that shooting, the Pentagon ordered tightened security at all U.S. bases.
Defense Secretary Chuck Hagel called Wednesday's shooting a "terrible tragedy" for a community that has too recently see that kind of "senseless violence."