FORT HOOD, Texas — A soldier opened fire at a U.S. Army base in Fort Hood, Texas on Thursday, unleashing a stream of gunfire that left 12 people dead and 31 wounded. Authorities killed the gunman, and apprehended two other soldiers suspected in the attack.

The shooting began around 1:30 p.m., Lt. Gen. Bob Cone said at a news conference. He said all the casualties took place at the base's Soldier Readiness Center, where soldiers who are about to be deployed or who are returning undergo medical screening.

"It's a terrible tragedy. It's stunning," Cone said.

Little was known about the three soldiers suspected of taking part in the attack. The soldier used two handguns, Cone said. It was not clear if the gunman had stopped to reload.

A graduation ceremony for soldiers who finished college courses while deployed was going on in an auditorium at the Readiness Center at the time of the shooting, said Sgt. Rebekah Lampam, a Fort Hood spokeswoman.

Greg Schanepp, U.S. Rep. John Carter's regional director in Texas, was representing Carter at the graduation, said John Stone, a spokesman for Carter, whose district includes the Army post.

Schanepp was at the ceremony when a soldier who had been shot in the back came running toward him and alerted him of the shooting, Stone said. The soldier told Schanepp not to go in the direction of the shooter, he said. Stone said he believes Schanepp was in the theater.

The base was locked down after the shootings. The wounded were dispersed among hospitals in central Texas, Cone said.

In Washington, President Barack Obama called the shooting "a horrific outburst of violence." He said it's a tragedy to lose a soldier overseas and even more horrifying when they come under fire at an Army base on American soil.

"We will make sure that we get answers to every single question about this horrible incident," the commander in chief said. "We are going to stay on this."

Covering 339 square miles, Fort Hood is the largest active duty armored post in the United States. Home to about 52,000 troops as of earlier this year, the sprawling base is located halfway between Austin and Waco.

Around the country, some bases stepped up security precautions, but no others were locked down.

Fort Hood officially opened on Sept. 18, 1942, and was named in honor of Gen. John Bell Hood. It has been continuously used for armored training and is charged with maintaining readiness for combat missions.

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AP National Security Writer Anne Gearan and Associated Press writers Lolita C. Baldor and Devlin Barrett in Washington, D.C., and Associated Press Writer Jay Root in Fort Worth, Associated Press Writers Linda Stewart Ball, Anabelle Garay and Andre Coe in Dallas contributed to this report.

Copyright 2009 The Associated Press.