Seven people have been killed and at least 20 injured in a pair of shootings at the Fort Hood military base in Texas, the US Army has confirmed.
One person has been arrested and at least one more is on the run, reports say. The base has been locked down.
NBC News network said the two suspects were in military uniform and that the shooter-at-large was believed to have a high-powered sniper rifle.
Fort Hood, near the town of Killeen, is the largest US base in the world.
Home to about 40,000 US troops, the base lies between Austin and Waco, about 60 miles (97 km) from each city.
It is not yet clear whether those reported killed and injured are civilians or military personnel.
White House spokesman Robert Gibbs said President Barack Obama had been briefed on the incident.
Army spokesman Lt Col Nathan Banks at the Pentagon told the Associated Press news agency the shootings had begun at about 1330 (1930 GMT) on Thursday at a personnel and medical processing centre at Fort Hood.
He said two shooters had been involved.
The second incident took place at a theatre on the base, he said.
NBC reports that the suspect in custody is in his 20s.
At this point all those involved are believed to be military personnel, ABC reports. It says there are conflicting reports about whether there is a third shooter.
A soldier stationed at Fort Hood told the BBC: “We’re on lockdown. I heard the emergency announcement over the speakers outside and saw people rushing to get indoors.
“In our office we’re okay but we’re hearing about the deaths. It’s horrible and very shocking.”
Senator Kay Bailey Hutchinson, talking on CNN, said she had spoken to one of the generals at Fort Hood minutes ago, and he had suggested 30 people were wounded.
Local congressman John Carter, speaking to NBC News, said gunfire had erupted during a graduation ceremony.
The BBC’s Adam Brookes in Washington says there are military police and Swat teams on the scene, and the FBI is on the way from Austin and Waco. Schools in the area have also been locked down.
The units at the base will be deployed in Iraq and Afghanistan, and some will have returned from there, our correspondent says.
The base is essentially like a small town, he adds. There is a centre there that deals with combat stress.
BBC