At least one American soldier went on a shooting rampage in a sprawling Texas military base killing 11 people and wounding 31 others before being gunned down, army officials said.
Two other soldiers were arrested after the shooting spree and were being held as suspects, said Lieutenant General Bob Cone, commander at Fort Hood, the largest US military base in the world.
The soldier, identified as Major Hassan Malik, on Thursday opened fire with two handguns in a busy processing centre where troops go before being deployed on overseas missions in Iraq and Afghanistan.
Cone told reporters the motive for the attack remained unclear. "We don't know that right now. Again, this is all under investigation," he stressed, adding he believed the incident was now over.
General Cone confirmed that 12 people were dead, including the shooter.
US President Barack Obama, who had been kept informed of the shooting as the drama was tracked in the White House situation room, denounced the attack as "an horrific outburst of violence."
"My immediate thoughts and prayers are with the wounded. And with the families of the fallen," Obama said.
Texas Senator Kay Bailey Hutchison said Malik was about to be deployed to Iraq.