Investigators delved into the background Thursday of a US soldier who had been treated for mental illness before he killed three people and himself a day earlier at an Army base in Fort Hood, Texas, where another deadly rampage took place in 2009, military officials said.
There was no motive given for the incident, although officials ruled out terrorism. The shooting also wounded 16 others, and the gunman, who had been treated for depression and anxiety, was yet to be officially identified.
The victims were all military personnel at the massive Army base, which houses many soldiers who have seen combat abroad.
The soldier went to two buildings on the base and opened fire before he was confronted by military police, Fort Hood commanding officer Lieutenant General Mark Milley said.
The gunman then shot himself in the head with a .45-caliber pistol, he said.
"At this time there is no indication that this incident is related to terrorism," Milley told a news conference.
The rampage is the third shooting at a military base in the United States in about six months that, along with a series of shootings in public places, such as schools and malls, has sparked a national debate over gun control regulations.
Security officials said preliminary information identified the gunman as Ivan Lopez but Milley declined to identify the shooter, who is married, until his family was notified.
The suspect's wife was cooperating with law enforcement officers, a Federal Bureau of Investigation official said, according to CNN.
The shooter had served for four months in Iraq in 2011, Milley said, and was undergoing evaluation for post-traumatic stress disorder. He arrived in Fort Hood, one of the largest US Army bases, in February from another military installation.
The Scott & White Hospital in Temple, Texas, where some of the wounded were taken, said nine patients were in intensive care, three in critical condition and six in stable condition. Other victims were taken to Fort Hood's Carl R. Darnall Army Medical Center, near where the shooting took place.
US President Barack Obama said he was "heartbroken" that another shooting had occurred at the Fort Hood Army base.
"We are going to get to the bottom of exactly what happened," Obama said. "We're heartbroken that something like this might have happened again."
The latest shooting at Fort Hood is throwing a spotlight on the US military's so-far frustrated efforts to secure its bases from potential shooters, who increasingly appear to see the facilities as attractive targets.
The shooting started at about 4 p.m. local time (2100 GMT) and put Fort Hood on immediate lockdown.
Milley said the shooter walked into one of the unit buildings, opened fire, then got into a vehicle and fired from there. He then went into another building and opened fire again, until he was engaged by Fort Hood law enforcement officers.
When confronted by a female military police officer, he shot himself with his semi-automatic weapon in the parking lot.
"He was approaching her at about 20 feet. He put his hands up, then reached under his jacket, pulled out the (.45) and she pulled out her weapon and then she engaged, and he then he put the weapon to his head," Milley said.
One of the buildings housed medical brigade day-to-day operations and the other, nearby, served the administration of the transportation battalion. All the dead and wounded were military personnel.
As soon as the shooting broke out, police secured the base perimeter, emergency vehicles rushed to the scene, helicopters circled Fort Hood and officers went from building to building searching for the shooter.
"We're camping out. ... The only guidance we've been given is to hunker down," a Fort Hood soldier who answered the phone at a building near the shooting told the Austin American-Statesman.
Central Texas College ordered an immediate evacuation of all students and staff and canceled classes at its Fort Hood campus.
"It's a terrible tragedy. We know that. We know there are casualties, both people killed and injured," Defense Secretary Chuck Hagel said.
The violence echoed the rampage of 2009, when a former army psychiatrist shot dead 13 people and wounded 32 others in a shooting spree at Fort Hood, a base from where soldiers prepare to deploy to Iraq and Afghanistan.
Major Nidal Hasan shouted "Allahu Akbar," Arabic for "God is greatest," during the attack and later said he wanted to be a martyr. He was convicted and faces death by lethal injection.
In February, the US military demolished the building where Hasan went on his shooting spree and plans to plant trees, install a gazebo and mark the site with a remembrance plaque.
"It was just like a kick in the gut. It made me sad, it made me angry. It made me want to do something to help," Killeen Mayor Daniel Corbin told CNN, responding to a question about another burst of violence on the sprawling base.
In September, a gunman opened fire at the Washington Navy Yard, killing 12 and wounding four before being killed by police. Last month, a civilian shot dead a sailor aboard a ship at a US Navy base in Norfolk, Virginia.
Hagel said the latest incident at Fort Hood showed that there were problems that still needed to be addressed.
Last month, he ordered steps to improve Pentagon security after reviews found the Navy Yard shooting could have been averted if the gunman's mental health had been properly handled.