Armed gunmen Thursday killed seven people and wounded at least 12 others in a rampage at Fort Hood military base in Texas, US media and officials said.

One gunman was in custody, but another remained on the loose on the sprawling base in Killeen, Texas, MSNBC television reported, adding the shooter at large was believed to have a high-powered sniper rifle. It added there was also speculation that there were possibly three gunmen.

Local congressman John Carter confirmed on MSNBC there had been a shooting at the base, saying the gunfire had erupted during a graduation ceremony in the soldier readiness center. He could not confirm reports of injuries.

Fort Hood is the largest US military base in the world and had been working to rehabilitate many soldiers suffering from post-traumatic stress syndrome, Carter added.

Army personnel told a Fox news affiliate they were not sure if the victims were civilians or military personnel.

A message on Fort Hood's website -- the headquarters of the Army 3rd Corps, the 4th Infantry Division and the 1st Cavalry Division -- said it was closed, but gave no further details. All those units have seen extensive duty in Iraq.

- Effective immediately. Fort Hood is closed. Organizations/units are instructed to execute a 100 percent accountability of all personnel. This is not a Drill. It is an Emergency Situation - the website.

Schools in the area were also on lockdown, MSNBC said.