A soldier opened fire at Fort Hood, an army base in Texas and scene of a massacre in 2009, killing three people and wounding at least 11 others before taking his own life.
The violence Wednesday apparently stemmed from some sort of soldier dispute, CBS News said citing law enforcement sources. The shooter had been identified as 34-year-old soldier Ivan Lopez, it said.
President Barack Obama who was in Chicago was briefed about the incident. Speaking to reporters at a hastily arranged briefing at Chicago Cut Steakhouse, he said: "The situation is fluid right now."
The President said his national security team was in close contact with the Defence Department and FBI and "working with folks on the ground to determine exactly what happened to make sure that everybody is secure".
"And I want to just assure all of us that we're going to get to the bottom of exactly what happened, he said.
"Any shooting is troubling. Obviously this reopens the pain of what happened at Fort Hood five years ago," Obama said recalling the 2009 shooting.
"We're heartbroken that something like this might have happened again."
Warning sirens went off around 4.25 p.m. and all personnel were urged to shelter in place as the base went on lockdown, according to CBS.
A man, who said he was a witness, told CBS affiliate KWTX that about 20 shots were fired at a post motor pool.
The man said he saw a soldier jumping over a fence and running away, but it was not clear whether that solider may have been the shooter.
Emergency crews from several surrounding communities headed to the base, home of the Army's 1st Cavalry Division.
Ford Hood near Killeen in central Texas was the site of a mass shooting Nov 5, 2009, when Major Nidal Malik Hasan, an Army psychiatrist, opened fire at a soldier readiness centre on the base.
He shot 13 people dead and wounded more than 30 others in what was the worst shooting ever to take place on an American military base.
Hasan, 43, who who said he was driven by a hatred of the wars in Iraq and Afghanistan, was left paralysed when he was shot by police responding to the shooting spree. He has been sentenced to death for the rampage.
