KILLEEN, Texas (CNN) -- Fort Hood.
The name has been seared in the nation's collective memory since a soldier went on a deadly shooting spree here in 2009.
On Wednesday, it happened again.
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.
The 34-year-old Iraq vet then put the gun to his head and pulled the trigger, ending his life.
Authorities say they have not ruled out terrorism, but they were downplaying the possibility.
"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's 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 apparently plagued by multiple mental health issues.
He had arrived at the base in February, moving with his wife and their daughter into an apartment a little more than a week before the shooting.
They appeared to be a normal couple, said neighbor Xanderia Morris. "They would smile whenever they'd see someone," she said.
But behind Lopez's smile lay a history of depression, anxiety and other psychiatric disorders, according to Milley, and he was receiving treatment and medication.
He had 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.
"He was not diagnosed, as of today, with PTSD," Milley said.
Arriving at the diagnosis common among war veterans can take time.
He used his own weapon
Lopez had been part of the National Guard in Puerto Rico, but he had left the Guard to join the U.S. Army, National Guard spokeswoman Ruth Diaz said Thursday. He carried out the killings with his own gun -- a .45-caliber Smith and Wesson semiautomatic pistol he bought after arriving in Killeen.
By taking 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 an armory.
Sequence of events
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, got out of the car, walked into another building and fired again.
Over 15 to 20 minutes, he killed three and wounded 16 -- all of them army personnel, Milley said.
Three of the wounded were in critical condition early Thursday.
Authorities did 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 logistics.
The base, which has more than 45,000 soldiers and nearly 9,000 civilian employees, went on lockdown.
People were told to shelter in place.
As sirens blared, Pvt. Dehlan Kay stayed in his barracks and talked on a phone. "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, the shooter's wife was watching the news.
She came out crying, worried about her husband, from whom she had not heard all afternoon. But she had no idea that he was the shooter, said Morris, the neighbor.
"I'm just worried, I'm just worried," Lopez's wife told her.
"I tried to console her and comfort her, let her know everything was OK," Morris said.
It was not. When 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.
The shooting spree ended when a female officer confronted Lopez in a parking lot; he reached under his jacket for his pistol, put it to his head and fired.
Nine of the wounded were taken to Scott & White Memorial Hospital in nearby Temple, where three were in critical condition and six were in serious condition, with the possibility of being upgraded later in the day to fair, said Dr. Matthew Davis.
"It's been a busy evening," the surgeon said early Thursday.
Two of the most seriously wounded were operated on overnight and will require further surgery, he told CNN. "The next 24 hours are really critical," he said.
Though some of the wounds were superficial, others involved vital structures -- the neck, the chest, the abdomen, he said. Though most of the wounded will heal, "we clearly are going to have some physical scars and emotional scars going forward," he said. He predicted that the actions of a man who himself may have had PTSD probably will be responsible for others experiencing it.
"We'll have to work with some of our long-term professionals who help to work with PTSD," he said.
The last mass shooter
In squeezing off his final shot, Lopez's actions differed from those of Fort Hood's last mass shooter.
Army Maj. Nidal Malik Hasan survived after killing 13 people and wounding another 32 on November 5, 2009.
Months later, the former military psychiatrist told a court that he was on a terrorist mission.
During a hearing in June, he said that he fired at soldiers preparing to deploy to Afghanistan to protect leading members of the Taliban.
Hasan was convicted of 13 counts of murder and 32 counts of attempted murder, and a military jury recommended last August that he be put to death.
"My reaction was not 'not again here,' " Milley said in response to reporters' questions. "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 saw the link.
"That was so similar, it just tore up my heartstrings," Staff Sgt. Patrick Zeigler told CNN's "New Day" on Thursday.
Zeigler, who is still recovering from four bullet wounds to the head and body from the 2009 Fort Hood shooting, expressed surprise. "It's just hard to believe that these people that you serve with, who are your brothers in arms, would turn against you," he said. "And it's just incredible that at home, someone in the environment of a military base would decide to do this."
But security procedures will never suffice to deter someone bent on committing such an act, he said. "It takes somebody brave enough to report these people in order for it to be prevented."
"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 defense and FBI leaders by phone while traveling on Air Force One.
Fort Hood has been resilient before, Gov. Rick Perry said.
CNN's Joshua Rubin reported from Killeen, Texas; Ben Brumfield and Tom Watkins reported and wrote from Atlanta. CNN's Dana Ford, Steve Almasy, Nicole Dow, Greg Botelho, Evan Perez, Shimon Prokupecz, Pamela Brown, Brian Todd, Matt Smith, Barbara Starr, Carma Hassan and Devon Sayers contributed to this report.