A shooting incident has taken place at the Texas Army base that was the site of a gun rampage in 2009 that left 13 soldiers dead and 32 wounded.
A senior US official told news agencies that one person was dead and at least 14 injured in the latest incident.
The shooting is said to have occurred at the Carl R Darnall Army Medical Center, located on the base.
President Barack Obama said he was "heartbroken" over the news and vowed to investigate fully.
Fort Hood's Directorate of Emergency Services told US media it had received reports, though unconfirmed, that the gunman was dead.
A number of people with injuries are being transported to area hospitals.
"We're following it closely," Mr Obama said, adding that this was a "fluid" situation.
"I want to just assure all of us we are going to get to the bottom of exactly what happened," he told reporters in Chicago.
"We're heartbroken something like this might have happened again."
Mr Obama said Ford Hood had sacrificed so much for freedom.
Maj Nidal Hasan was sentenced to death last September over the 2009 shooting rampage at the same facility.
He claimed he opened fire at the army base to protect Taliban insurgents from US troops who were about to deploy to Afghanistan.
It was the deadliest attack on a domestic military installation in US history.
Fort Hood all over again
Fort Hood all over again
“If you trust in yourself, and believe in your dreams, and follow your star. . . you'll still get beaten by people who spent their time working hard and learning things and weren't so lazy.”