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No motive was given for the shooting spree, which also left 16 wounded in what was the second mass killing in five years at one of the largest military bases in the United States, raising questions about security at such installations. Officials have so far ruled out terrorism.
"We have very strong evidence that he had a medical history that indicates unstable psychiatric or psychological conditions," Lieutenant General Mark Milley told reporters.
"There may have been a verbal altercation with another soldier or soldiers. There is a strong possibility that that in fact immediately preceded the shooting," said Milley, adding there was no indication that he targeted specific people.
Lopez, 34, originally from Puerto Rico, had been treated for depression and anxiety. He was being evaluated to see if he suffered from post-traumatic stress disorder, or PTSD, military officials said.
He is suspected of smuggling onto the base a recently purchased Smith & Wesson .45 caliber pistol that was used in the shootings.
Milley said Lopez purchased the firearm at Guns Galore, the same store in Killeen where former Army psychiatrist Major Nidal Hasan bought the weapon he used to kill 13 people and wound 32 others at Fort Hood in 2009.
U.S. Army Secretary John McHugh said Lopez, who joined the service in 2008, had served two tours of duty abroad, including four months in Iraq in 2011. He had no direct involvement in combat and had not been wounded.
"He was undergoing a variety of treatment and diagnoses for mental health conditions, ranging from depression to anxiety to some sleep disturbance. He was prescribed a number of drugs to address those, including Ambien," McHugh told a U.S. Senate committee hearing.
Lopez served in the Puerto Rico National Guard for several years in an infantry unit and as a band member, both military combat training assignments; he also did a stint as part of an observation mission in the Sinai, Egypt, Puerto Rico National Guard Major Jamie Davis told Reuters.
Three of the soldiers listed in critical condition were showing signs of improvement and their condition was upgraded to serious, a doctor at Scott & White hospital in Temple told CNN.
One of the injured was identified by his family via Twitter as Major Patrick Miller, of New York.
At the modest blue-and-gray apartment building in Killeen where Lopez lived with his wife and 2-year-old daughter, American flags flew and "Welcome home" signs adorned the walls of a place favored by soldiers rotating through the base.
Army chaplains visited the family on Thursday.
Shaneice Banks, 21, a self-described Army wife, was with Lopez's wife when news of the shooting broke.
"She heard her husband's name on the news and she just lost it," Banks told Reuters.
Another neighbor, Mahogoney Jones, 21, said the wife was in a state of panic. "She's calling and calling her husband because she feels something is wrong. She kept screaming 'No answer! No answer!'".
Jones said she last saw Lopez when he came home for lunch on the day of the shooting.
"He was calm. He petted my dog and then went back to base," she said.
There are about 45,000 soldiers and airmen assigned to the 335-square-mile (870-square-km) base along with nearly 10,000 civilian employees, according to Fort Hood.
It was not clear what spurred the gunman to enter two base buildings and open fire on fellow soldiers at about 4:00 p.m. local time (2100 GMT) on Wednesday.
When confronted by a female military police officer in a parking lot, he killed himself with his semi-automatic weapon.
The incident is the third shooting at a military base in the United States in about six months that, along with a series of shootings in schools and malls, has sparked a national debate over gun violence.
Retired Army Sergeant Alonzo Lunsford, who was shot multiple times in the 2009 incident at Fort Hood, said the military has not done enough to treat the mental scars of those who have served in combat regions.
"The military needs to go ahead and stop talking about the problem and talking about what we're going to do. Just do it," Lunsford said.