FORT HOOD -- Fort Hood officials confirmed on Thursday they were investigating the strong possibility that an argument between Specialist Ivan Lopez and another soldier or soldiers was the trigger event that prompted the truck driver to open fire and kill three Wednesday.

There is a strong possibility that immediately preceded the shooting, Lt. Gen. Mark Milley said at a Thursday afternoon briefing.

However, there's no indication that he targeted specific soldiers, Milley said.

He also confirmed that Lopez only recently transferred to the post from Fort Bliss.

The post commander said the transfer had nothing to do with seeking treatment at Fort Hood, but did acknowledge investigators were thoroughly examining Lopez's behavioral and mental problems.

The 34-year-old truck driver from Puerto Rico seemed to have a clean record that showed no ties to extremist groups. But the Army secretary promised that investigators would keep all avenues open in their inquiry of the soldier whose rampage ended only after he fired a final bullet into his own head.

Late Thursday night, the military released Lopez's official military record. It includes some recognitions and awards, including two Army Commendation Medals and four Army Achievement Medals.

We're going to keep an open mind and an open investigation. We will go where the facts lead us, Army Secretary John McHugh said, explaining that possible extremist involvement is still being looked at very, very carefully.

Lopez grew up in Guayanilla, a town of fewer than 10,000 people on the southwestern coast of Puerto Rico, with a mother who was a nurse at a public clinic and a father who did maintenance for an electric utility company.

Glidden Lopez Torres, who said he was a friend speaking for the family, said Lopez's mother died of a heart attack in November.

The soldier was upset that he was granted only a 24-hour leave to attend her funeral, which was delayed for nearly a week so he could be there, the spokesman said. The leave was then extended to two days.

Lopez joined the island's National Guard in 1999 and served on a yearlong peacekeeping mission in Egypt's Sinai Peninsula in the mid-2000s. He enlisted with the Army in 2008 and saw no combat during a four-month deployment to Iraq as a truck driver in 2011, McHugh said.

Investigators searched the soldier's home Thursday and questioned his wife, Fort Hood spokesman Chris Haug said.

Neighbors of Lopez said they were stunned to find out the 34-year-old was behind the attack.

Seeing his picture now, it's just very hard, said Shiniece Banks, fighting back tears.

Suzie Miller, a 71-year-old retired property manager who lived in the same Killeen apartment complex as Lopez, said few people knew him and his wife well because they had just moved in.

I'd see him in his uniform heading out to the car every morning, Miller said. He was friendly to me and a lot of us around here.

Flags in and around Killeen and on post were lowered to half-staff to honor the three killed and 16 wounded.

Post officials also said an Army chaplain helped shield some soldiers from Lopez's actions. Milley said there were quite a few heroes that stepped forward during the 20-minute shooting ordeal.

Scott & White Memorial Hospital in nearby Temple, Texas, was still caring for several of the 16 people who were wounded. All of them were in either serious or good condition, and some could be discharged before the end of Thursday.

Hospital officials had no information about patients being treated elsewhere, including at a base hospital. But because Scott & White is the area's only trauma center, the patients with the most serious injuries were probably taken there.

Officials expected to give another briefing on Friday.

A memorial service is tentatively being scheduled for early next week.

The Associated Press contributed to this story.

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