Killeen, Texas (CNN) --Fort Hood shooter Ivan Specialist Ivan Lopez went from one building at the sprawling Texas military base to a second, firing a .45 caliber handgun -- killing three people and wounding 16 more. Lopez was part of the National Guard in Puerto Rico, National Guard spokeswoman Ruth Diaz said Thursday. Then the 34-year-old Iraq vet put the gun to his head and pulled the trigger, ending his life and taking with him the reasons for his action.
"There are initial reports there may have been an argument in one of the unit areas," Lt. Gen Mark Milley, the post's commanding general, told reporters late Wednesday.
Officers picked up Lopez' wife at their apartment near the base in Killeen, and she was cooperating with law enforcement, an FBI official told CNN.
The man, whom a neighbor said often gave her a friendly wave, was plagued by multiple mental health issues.
Lopez was new on the base, having only arrived there in February. He, his wife and their small daughter moved into their apartment a little more than a week before the shooting.
They were a normal couple, said neighbor Xanderia Morris. "They would smile whenever they'd see someone," she said.
But behind Lopez' smile lay intense emotional torment -- of depression, anxiety and other psychiatric disorders. He was receiving treatment and medication, Milley said.
He served for four months in Iraq in 2011. And while army records don't show him as having been wounded there, Lopez himself reported that he had suffered a traumatic brain injury, Milley said.
He was undergoing diagnosis procedures for Post-Traumatic Stress Disorder (PTSD).
"He was not diagnosed, as of today, with PTSD," Milley said.
Arriving at the diagnosis of the mental ailment that plagues so many war veterans takes time.
When he took it onto the base, he was breaking the rules.
"If you have weapons and you're on base, it's supposed to be registered on base," Milley said. "This weapon was not registered on base."
In addition, people are not allowed to walk around with guns on a military base. They are required to store them in a secured armory.
The exact sequence of events is not entirely clear. But around 4 p.m., Lopez walked into a building at the base and opened fire. He then got into a car, fired from the vehicle, walked into another building and fired again.
He killed three and wounded 16 -- all of them were army personnel.
Three of the wounded remained in critical condition early Thursday morning.
Authorities could not say whether Lopez knew his victims.
The shootings took place in the medical brigade and the transportation battalion buildings. Lopez was assigned to the 13th sustainment command, which deals with the logistical responsibilities for the post.
The spree went on for about 15 to 20 minutes, Milley said.
The base housing more than 45,000 soldiers and nearly 9,000 civilian employees went on lockdown.
People were told to shelter in place.
Pvt. Dehlan Kay stayed in his barrack, as sirens went off.
"I'm doing good," he told CNN's Wolf Blitzer. "I'm just a little nervous on what's happening."
The all-clear wouldn't go out for another six hours.
At the Lopez apartment, his wife was watching reports of the shooting on TV.
She came out crying, said Morris, the neighbor.
"I'm just worried, I'm just worried," Lopez' wife told her. She hadn't heard from her husband all afternoon.
"I tried to console her and comfort her, let her know everything was OK," Morris said.
They had no idea who the shooter was at the time.
Then, a local TV station identified the gunman as Lopez.
The wife became "hysterical," Morris said.
It took law enforcement about 15 minutes to respond to the gunfire, Milley said.
An officer confronted Lopez in a parking lot.
He reached under his jacket for his pistol, and put it to his head. He fired. Death by self-inflicted gun shot wound.
"It was clearly heroic what she did," Milley said of the offier. "She did her job and she did exactly what we would expect of the U.S. military police."
Here, Lopez' actions differed from Fort Hood's last mass shooter.
Army Maj. Nidal Malik Hasan survived after killing 13 people and injuring another 32 on November 5, 2009.
The former military psychiatrist openly told a court that he was on a terrorist mission.
During a hearing he said that he fired on soldiers preparing to deploy to Afghanistan to protect leading members of the Taliban.
Hasan was convicted of premeditated murder, and a military jury recommended that he be put to death.
Reporters prodded Milley about the 2009 shooting and Lopez' rampage -- a repeat tragedy.
But he would not entertain the notion.
"My reaction was not 'not again here,'" he said. "My reaction was to immediately make sure we had a read on the casualties. Immediately secure the site. Immediately look for one or more shooters."
But others were more introspective.
"As a community, it's like you've been kicked in the gut. It can't be happening again," said Killeen Mayor Dan Corbin.
No community should have to experience such violence once, let alone twice, said John Cornyn, a U.S. senator from Texas.
"We're heartbroken something like this might have happened again," said President Obama, who was briefed by top defense and FBI leaders by phone while traveling on Air Force One.
Fort Hood has been resilient before, Gov. Rick Perry said. And it will again.