A shooting has left at least four people dead, including the gunman, and 16 injured at a US Army base in Fort Hood, Texas, the site of another mass shooting in 2009, the US military said.
Fort Hood commander Lt Gen. Mark Milley told a news conference that the shooter was a serving soldier and that he had died on the scene from self-inflicted gunshot wounds after being confronted by a military police officer.
Milley said there was no indication the incident was linked to "terrorism".
He told reporters the soldier, who he did not name because his family had not yet been informed, had served in Iraq for four months in 2011. The soldier was being treated for depression and anxiety, and was being evaluated for possible post-traumatic stress disorder.
Milley did not draw a link between the gunman's medical evaluation and the shooting.
The Associated Press reported that the soldier had shot himself in the head after being confronted by military police in a car park at the base, after entering two buildings and firing.
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As he came within six metres of an officer, he put his hands up but then reached under his jacket and pulled out his gun. The officer drew her own weapon but the suspect put the gun to his head and pulled the trigger, the agency reported.
NBC correspondent Jay Gray told Al Jazeera that it was believed a.45 calibre Smith and Wesson pistol used in the shooting had been recently purchased from the local area and was not a military weapon.
Investigators, including the Federal Bureau of Investigation, army investigators, state and local police, were examining whether the incident was related to a disagreement that had escalated between two soldiers, Gray said.
The suspect's wife was cooperating with investigators, an FBI official said, according to CNN.
In an inititial response to the attack, US President Barack Obama promised a full investigation.
"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."
The Scott & White Hospital in Temple, Texas, where some of the wounded were taken, said nine patients were in intensive care and three in a critical condition. Other victims were taken to Fort Hood's Carl R. Darnall Army Medical Center, near where the shooting occurred.
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Some of the victims suffered single and multiple gunshot wounds, including injuries to the abdomen, chest and neck.
Al Jazeera's Rosalind Jordan, reporting from Washington DC, said there were chaotic scenes around the base in the hours after the shooting, with numerous helicopters and ambulances at the site.
"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 military base," Jordan said.
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.
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.