Monica Cain, wife of soldier Darren Cain, talks to family and friends while waiting outside Ft. Hood on November 5, 2009 in Killeen, Texas. At least one gunman killed 12 people and injured 31 in a shooting on a military base at Ft. Hood this afternoon.[CFP]

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.

A law enforcement official identified the shooting suspect as Army Maj. Malik Nadal Hasan. The official said Hasan, believed to be in his late 30s, was killed after opening fire at the base. The official spoke on condition of anonymity because he was not authorized to discuss the case publicly.

The official says investigators are trying to determine if Hasan was his birth name, or if he changed his name and converted to Islam at some point in his life.

Cone said the soldier used two handguns in the attack. 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, US 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.

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

The shootings on the Texas military base stirred memories of other recent mass shootings in the United States, including 13 dead at a New York immigrant center in March, 10 killed during a gunman's rampage across Alabama in March and 32 killed in the deadliest mass shooting in modern US history at Virginia Tech in 2007.

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

"The bottom line for us is that we are increasing security at our gates because the threat hasn't yet been defined, and we're reminding our Marines to be vigilant in their areas of responsibility," said Capt. Rob Dolan, public affairs officer for the Marine Corps Air Station in Yuma, Ariz.

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.

About a mile from Fort Hood's east gate, Cynthia Thomas, director of Under the Hood Cafe, a local coffee shop and nonprofit military support center, has been calling soldiers and friends on the post to make sure they're OK.

"It's chaotic," Thomas said, as a SWAT team just drove by. "They're just saying that they're under attack they don't know what's going on. ... The phones are jammed. Everybody is calling family members and friends. Soldiers are running around with M-16s."

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.