The man who opened fire at the Fort Hood military post in Texas was a veteran who served four months in Iraq and was undergoing treatment for mental health issues.
Spc. Ivan Lopez, married with children, moved to the post in Killeen in February from another military installation.
Authorities don't know what prompted the shooting spree Wednesday, where Lopez, 34, killed three people, wounded 16 - before fatally shooting himself.
There aren't indications that this was a terrorist act - but officials said they won't rule anything out until the investigation is over.
"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.
"Obviously, we are digging deep into his background, any criminal or psychiatric history, his experiences in combat. All of the things you would expect us to do are being done right now."
Based on publicly released details, interviews with neighbors and conversations with law enforcement and other sources, here's what we know so far about Lopez:
He served for four months in Iraq in 2011. "He was not wounded, according to our records," Milley said. However, Lopez "self-reported" suffering a traumatic brain injury while deployed, he said.
Lopez suffered from depression, anxiety and other psychiatric complaints and was receiving treatment and medication. He was going through the process required to diagnose Post-Traumatic Stress Disorder (PTSD). "He was not diagnosed, as of today, with PTSD," Milley said. That process takes time.
Lopez was transferred to Fort Hood from another unnamed base in February. He was assigned to the 13th sustainment command, which deals with the logistical responsibilities for the post. (It was one of two unit buildings where Lopez opened fire).
Retired Army Maj. Gen. James "Spider" Marks expressed surprise at the transfer. Lopez should have remained at the other base for continuity of care, he said.
Lopez was not in the process of being transitioned out of the military, Milley said.
He was married and had a daughter, around 3 years old. Just over a week ago, the family moved in to an apartment complex close to the base.
Neighbor Xanderia Morris described the Lopez's as a 'typical, average family."
"They would smile whenever they'd see someone," she said.
After the news of the shooting broke on television, the wife came out crying. "She said 'I'm just worried, I'm just worried,'" Morris said. "I tried to console her and comfort her, let her know everything was OK."
When television reports identified the shooter as Lopez, the wife was "hysterical," the neighbor said.
She was taken from the apartment by law enforcement officials, and was cooperating, an FBI source told CNN.
Lopez used a .45 caliber Smith and Wesson semiautomatic pistol that he recently purchased in the area, Milley said. He didn't know how much ammunition Lopez was carrying.
"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."
That's the big unknown.
"There's no indication that this incident is related to terrorism although we are not ruling anything out and the investigation continues," Milley said.
Could it have been an argument? "There are initial reports there may have been an argument in one of the unit areas, but no indication of an argument at the WTU," Milley said. WTU is the acronym for the Warrior Transition Command, where wounded, ill and injured soildiders are taught resilience skills.
He also couldn't say whether Lopez knew his victims.