A shooting at Fort Hood on Wednesday left four people dead, including the gunman, and others critically injured, officials said.

The shooter, identified as 34-year-old soldier Ivan Lopez, died of a self-inflicted wound, the Justice Department said.

A US law enforcement official, citing official internal Justice Department updates, says 14 others were hurt.

The officials spoke on condition of anonymity because they were not authorized to release information by name.

However, the base said in a statement: "The number of injured are not confirmed at this time."

Four of the victims were transported to Scott & White Hospital with gunshot wounds, and two others in route. “Their conditions range from quite stable to quite critically injured," Glen Couchman, Scott & White's chief medical officer, said. Couchman said the hospital was not currently in need of blood donations from individuals.

Local news reports said there may be two suspected shooters, although this could not be confirmed.

US President Barack Obama said he was "heartbroken" that another shooting had occurred at the Fort Hood Army base and described the situation there as fluid.

"We are going to get to the bottom of exactly what happened," Obama told reporters in Chicago, where he is traveling for Democratic fundraisers. "We're heartbroken that something like this might have happened again."

After the shooting, the third such incident at a military base in the United States in about six months, Fort Hood was put on lockdown and police were securing its perimeter, military officials said. Helicopters could be seen circling the base in live TV footage broadcast from the scene.

A video from Fort Hood broadcast on local TV station KCEN showed a loud announcement going through the base telling people to lock their windows and doors.

Police went from building to building, while scores of police vehicles and ambulances were on the scene, the station said.

Authorities advised those on the post to stay away from windows and to keep doors closed and locked.

Central Texas College, which has a Fort Hood campus, ordered an immediate evacuation of all students and staff, and canceled classes.

In 2009, a former Army psychiatrist shot dead 13 people and wounded 32 others in a shooting spree at Fort Hood.

In September, a gunman opened fire at the Washington Navy Yard, killing 12 and wounding four before being slain by police. Last month, a civilian shot dead a sailor aboard a ship at a US Navy base in Norfolk, Virginia.