Two gunmen in military uniforms shot and killed as many as nine people and wounded as many as 20 at Fort Hood in Texas on Thursday, officials said.
One of the shooters has been apprehended, Fort Hood spokesman Sgt. Maj. Jamie Posten told CNN.
"At this point we're looking for the other shooter," Posten said. Asked for a description, he said, "we're trying to develop that information."
The shooters were wearing military uniforms, but it was unclear whether they were soldiers, said U.S. Sen. Kay Bailey Hutchison, R-Texas.
One of the gunmen was shot and wounded by military police, Hutchinson told CNN. She said she got her information from a general on the base.
Rep. John Carter, R-Texas, said between 20 and 30 were wounded.
President Obama has been informed of the incident, Press Secretary Robert Gibbs told reporters.
Officials at Fort Hood, which is the Army's largest U.S. post, were asking people there to stay away from windows, CNN affiliate KXXV said. The incident took place at the sports dome, now known as the soldier readiness area, the station reported.
FBI agents are headed to the scene to assist, said Erik Vasys, spokesman for the FBI office in San Antonio. He had no other details.
On the Fort Hood Web site, the word "closed" is posted with the statement, "Effective immediately, Fort Hood is closed. Organizations/units are instructed to execute a 100 percent accountability of all personnel."
Fort Hood, with about 40,000 troops, is home to the Army's 1st Cavalry Division and elements of the 4th Infantry Division, as well as the 3rd Armored Cavalry Regiment and the 13th Corps Support Command. It is located near Killeen, Texas.
At least 25,000 people are at Fort Hood on any given day, an Army spokesman at the Pentagon said.
Fort Hood is home to the Warrior Combat Stress Reset Program, which is designed to help soldiers overcome combat stress issues.