(CNN) -- Spc. 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 post in Texas on Wednesday.
Lopez took his own .45-caliber handgun onto the sprawling facility and killed three people and wounded 16 more before taking his own life. His death left authorities to piece together what in his background and medical treatment could have triggered a new round of bloodshed at the same Killeen post where an officer killed 13 people in 2009.
Lt. Gen. Mark Milley, the post's commanding general, told reporters Friday that investigators "do not believe" that Lopez's "underlying medical conditions ... are the direct precipitating factor" in the attack.
"The immediate precipitating factor was more likely an escalating argument in his unit area," Milley said.
Authorities have "credible information" that Lopez "was involved in a verbal altercation with soldiers from his unit just prior to him allegedly opening fire," Chris Grey, spokesman for the U.S. Army Criminal Investigation Command, said Friday.
In Guayanilla, a tight knit municipality on the southern coast of Puerto Rico, the shooting by a member of one of the town's best-known families shocked the seaside community of 22,000. Lopez hailed from a family of music lovers and churchgoers, played in a popular marching band and once served on the island's police force.
In a statement Friday, Lopez' father, also named Ivan, asked for prayers for all those affected. He described the soldier as "a calm family man" and "a good son."
"My son could not have been in his right mind," he said. "He was not like this."
Before Wednesday's shooting, Lopez stopped at the post's personnel office to pick up a leave form, according to the sister of one of the soldiers injured in the attack.
Armetra Otis, sister of Sgt. Jonathan Westbrook, said on CNN's "The Lead" that her brother "was at work and a guy came in and asked for a leave form."
The soldier was told he would have to come back later, Otis said.
"And apparently I guess he didn't want to hear that, so he came back and just opened fire, " Otis said.
Westbrook was shot four times, but released from a hospital Friday, his sister said.
Law enforcement sources told CNN that investigators were searching for possible motives, including whether Lopez was angry over canceled leave.
Milley said Friday that he "does not know for certain" whether Lopez met with superiors about a leave request.
If Lopez was seeking a leave this week, it wouldn't be his first.
Glidden Lopez Torres, a family spokesman, said Lopez' mother, Carmen, an emergency room nurse in their hometown, died of a heart attack in November. A month earlier, Lopez' grandfather had died in Guayanilla. The spokesman, who is not related to the soldier, said Lopez attended the funeral but was disappointed that it took about five days for his 24-hour leave to be approved by the military.
"The reality is that the death of his mother was unexpected and soldiers are usually given permission to travel home to the family," Lopez Torres said. "But the process in Ivan's case took some time. He arrived five days after his mother died... He was a little disappointed that it took so long for him to be granted a leave."
Lucy Caraballo, a Lopez family friend in Guayanilla, said the family put off the wake for days.
"They waited various days because it took Ivan a long time to get here," she said. "We didn't know when he was going to arrive."
Asked about that leave, Milley said Thursday: "I don't have details on that right now."
In Friday's statement, the soldier's father said that his son's medical treatment, the recent deaths of his mother and grandfather and changes related to a transfer of military bases "surely affected his condition."
The younger Lopez, a 34-year-old specialist, served four months in Iraq and was undergoing treatment for mental health issues.
He was married with a young child and moved to the post in Killeen in February from Fort Bliss, according to a Pentagon official. He moved into an apartment there with his wife and their daughter a little more than a week before the shooting.
Neighbor Ayesha Bradley told CNN she saw Lopez Wednesday afternoon. "He seemed pretty fine, happy," she said. "He didn't seem like, you know, like the type that would do what he did."
Lopez later reportedly paid his rent and added his wife's name to the lease.
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 of terrorism as a motive but officials said they won't rule anything out until the investigation is over. All evidence thus far indicates Lopez "acted alone," Grey said.
Milley said there were reports of an argument involving Lopez, and the "trigger event" could have been a possible altercation with a fellow soldier "that immediately preceded the shooting."
Based on publicly released details, interviews with neighbors and conversations with law enforcement and other sources, this is 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. 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, without identifying Lopez by name.
Lopez enlisted in the Army in April 2010 an infantry soldier, later becoming a truck driver.
Authorities looking into Lopez's combat experience in Iraq "so far ... have not discovered any specific traumatic event, wounds received in action, contact with the enemy or anything else specific that he may have been exposed to while deployed," Milley said Friday.
However, Lopez "self-reported" suffering a traumatic brain injury while deployed, Milley said.
Lopez was undergoing a variety of treatments for conditions including depression, anxiety and sleep disturbances, officials said. He was prescribed drugs that included Ambien, a sleep aid. Lopez was fully examined last month by a psychiatrist.
"We have very strong evidence that he had a medical history that indicates an unstable psychiatric or psychological condition. (We're) going through all records to ensure that is, in fact, correct. But we believe that to be the fundamental underlying causal factor," Milley told reporters Thursday.
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 Fort Bliss in El Paso in February. From April 2010 to February, he was stationed at Fort Bliss.
At Fort Hood, 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."
At Immaculate Conception church in Guayanilla, Caraballo said the Lopez family was active in church affairs and in the choir.
"Ivan was a good person, with a lot of values," she said. "What happened? We don't know what went through his head or what he went through recently in the Army. But he came from a good family. This is a very painful thing for us."
Edgardo Arlequin, Guayanilla's mayor, said he directed the school band where Lopez was a percussionist.
"I never had problems with him," he said. "He was talented and came from an extraordinary home... He was extraordinary human being."
Lopez 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 first news of a shooting broke on television, Lopez' 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.
Investigators "have not uncovered any history of criminal convictions or previous criminal activity by Spc. Lopez," Grey told reporters Friday.
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 the 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.
Guns Galore was the same place where Army Maj. Nidal Malik Hasan, Fort Hood's previous mass shooter, purchased his weapon.
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 Unit, where wounded, ill and injured soldiers are taught resilience skills.
Investigators "have not uncovered any history of criminal convictions or previous criminal activity by Spc. Lopez," Grey said Friday.
He added, "At this time, we have not established a concrete motive."
CNN's Evan Perez, Ed Lavandera, Pam Brown, Josh Rubin, Meredith Edwards, Dania Alexandrino and Barbara Starr, Miguel Marquez and Kevin Flower contributed to this report.