At least one person has been killed and 14 injured after a gunman opened fire in the Fort Hood US Army base in central Texas, a senior military official said, according to an Associated Press news agency report.
In an intitial response to the attack, US President Barack Obama described the situation as still fluid.
“I want to just assure all of us we are going to get to the bottom of exactly what happened,” he said in remarks distributed to the media. “We’re heartbroken something like this might have happened again.”
He added: “Many of the people there have been on multiple tours of Iraq and Afghanistan, they served with valour, they served with distinction. At their home base they need to feel safe. We don’t yet know what happened tonight but obviously that sense of safety has been broken once again.”
US defence minister Chuck Hagel said that people had been killed and wounded.
“It’s a terrible tragedy. We know that. We know there are casualties, both people killed and injured,” Hagel told a news conference in Honolulu, where he was meeting with Asian defence ministers.
“We don’t have all the facts yet. We will get those. It’s still under investigation,” he added.
Four patients ranging from stable to critical condition were transported to a Texas hospital following the shooting, the hospital said.
Two more patients were expected to arrive shortly, Glen Couchman, chief medical officer at Scott & White hospital in Temple, Texas, told reporters.
He said that victims suffered single and multiple gunshot wounds, including injuries to the abdomen, chest and neck.
Earlier, Fort Hood had sent out an order for its personel to “shelter in place” on the base’s Twitter feed and also posted it on its Facebook page.
Warnings were also broadcast on loud speakers in the surrounding neighbourhood.
The 1st Calvary Division, which is based at Fort Hood, sent a Twitter alert telling people on base to close doors and stay away from windows.
There were conflicting reports in the US media about whether there was one gunman or two.
Fort Hood is one of the most heavily populated and used army bases in the US and is known for sending more troops to Iraq than any other US miltary base.
There were chaotic scenes around the base with multiple helicopters and ambulances at the site.
A video from Fort Hood broadcast on local TV station KCEN TV showed a loud announcement going through the base telling people to lock their windows and doors.
Police in the city of Waco, close to Fort Hood, tweeted that people should stay indoors, take shelter and monitor the media.
In the 2009 incident, 13 people were killed and more than 30 wounded in what was the deadliest assault on a domestic US military installation in history.