(CNN) -- Ivan Lopez's friendly smile apparently gave no hint of a history of depression, anxiety and other psychiatric disorders.
The Iraq war veteran was being evaluated for post-traumatic stress disorder before he opened fire at the Fort Hood Army base in Texas on Wednesday. He took his own .45-caliber handgun onto the sprawling base and killed three people and wounded 16 more before taking his own life, leaving authorities to piece together what in his background and medical treatment could have triggered a new round of bloodshed at the same Killeen base where an officer killed 13 people in 2009.
Lopez, a 34-year-old specialist, served four months in Iraq and was undergoing treatment for mental health issues.
A native of Puerto Rico, he was married with a young child and moved to the post in Killeen in February from another military installation. He moved into an apartment there with his wife and their daughter 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.
There aren't indications that this was a terrorist act, but officials said they won't rule anything out until the investigation is over.
Lt. Gen. Mark Milley, the post's commanding general, told reporters late Wednesday there were reports of an argument.
"Obviously, we are digging deep into his background, any criminal or psychiatric history, his experiences in combat," he said. "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 is known so far about Lopez:
He served for four months as a truck driver in Iraq in 2011, Army Secretary John McHugh told the Senate Armed Services Committee on Thursday, without naming Lopez. His records "show no wounds, no direct involvement in combat ... or any injury that might lead us to further investigate battle-related TBI (traumatic brain injury)," McHugh said. Lopez enlisted in the Army in June 2008 as an infantry soldier, later becoming a truck driver, McHugh said.
Milley said, "He was not wounded, according to our records." However, Lopez "self-reported" suffering a traumatic brain injury while deployed, he said.
Lopez was undergoing a variety of treatments for conditions including depression, anxiety and sleep disturbances, McHugh said. He was prescribed drugs that included Ambien, a sleep aid. Lopez was fully examined last month by a psychiatrist.
There was no record of any sign he was likely to commit violence against himself or others, according to McHugh. "So the plan (going) forward was just to continue to monitor and treat him as deemed appropriate," he said.
He was going through the process required to diagnose post-traumatic stress disorder. "He was not diagnosed, as of today, with PTSD," Milley said. That process takes time. Milley said Lopez was also taking antidepressants
"He was not a wounded warrior, no Purple Heart, not wounded in action in that regard," Milley said.
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 about 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.
At one time, he was part of the National Guard in Puerto Rico, but he left the Guard to join the Army, National Guard spokeswoman Ruth Diaz said Thursday.
Diaz said Lopez was active in the National Guard from 1999 to 2010. He was first assigned to the infantry battalion and later deployed, in 2007, to the Sinai Peninsula for 13 months. He joined the active duty Army in 2010.
Puerto Rico National Guard Command Sgt. Maj. Nelson Bigas, who served with Lopez for 17 months during training at Fort Hood and later in the Sinai, described Lopez as "one of the best soldiers we had in our infantry."
He told CNN that Lopez displayed no signs of mental issues or PTSD after completing his mission and returning to the island. "During, before, and after the mission, he never showed any signs of distress," said Bigas, adding that Lopez worked hard and demonstrated leadership within his team.
Bigas said the Fort Hood shooting was a "big surprise to me."
He was married and had a daughter, around 3 years old. Just over a week ago, the family moved into an apartment complex close to the base.
Neighbor Xanderia Morris described the Lopez family as a "typical, average family."
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 became "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 & Wesson semiautomatic pistol that he had recently purchased, 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."
Lopez passed a background check when he bought a .45-caliber Smith & Wesson handgun at Guns Galore near Killeen, Texas, according to U.S. law enforcement officials. He was found to have no criminal history that would disqualify him from the purchase, and the gun store did what was required, according to law enforcement officials who reviewed the records.
It appears that military doctors treating Lopez had not declared him mentally unfit in ways that would require reporting him to the FBI-run National Instant Criminal Background Check System, known as NICS.
The federal background check system relies on state and federal authorities to provide data on people who are mentally ineligible to buy firearms. Texas law sets a high bar to deny firearms purchases, including having been diagnosed by a licensed physician as suffering from a psychiatric disorder that is likely to cause substantial impairment in judgment and intellectual ability.
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 soldiers are taught resilience skills.
He also couldn't say whether Lopez knew his victims.