HOUSTON, April 3 -- One day after a U.S. soldier killed three people and wounded 16 others at the Army post of Fort Hood in Texas, authorities on Thursday are trying to piece together what in his background could have motivated the bloodshed though much still remained a mystery.

The gunman was identified by Congressman Michael McCaul, chairman of the House Homeland Security Committee, as 34-year-old Ivan Lopez, a Puerto Rican, who was active-duty on the post, working as a truck driver. Lopez allegedly opened fire Wednesday afternoon at two buildings in Fort Hood, killing three soldiers and wounding 16 more before killing himself.

Army Secretary John McHugh told the Senate Armed Services Committee on Thursday, without naming Lopez, that the gunman had a four-month stint in Iraq as a truck driver and that he was not involved directly in any combat there.

His records "show no wounds...or any injury that might lead us to further investigate battle-related TBI (traumatic brain injury), " McHugh said.

Lt. Gen. Mark A. Milley, Commander of Fort Hood said late Wednesday at a news conference, "He was not wounded, according to our records." However, Lopez "self-reported" suffering a traumatic brain injury while deployed, Milley said.

However, McHugh said Lopez was undergoing a variety of treatments for conditions including depression, anxiety and sleep disturbances. He was prescribed drugs that included Ambien, a sleep aid. Lopez was fully examined last month by a psychiatrist.

There appeared to be no conspicuous motive for Lopez to open fire against his own comrades judging from his personal background and medical treatment. Milley said there was no indication that the shooting was related to terrorism, though they will not rule that possibility out until the investigation is over.

Some anonymous sources said on Wednesday the shooting might be resulted from disputes. Milley said there were reports of an argument but it didn't appear to be in the areas where actual shootings took place.

It's also unknown if any victims knew Lopez in person. Authorities have not released the identities of anyone of them.

Wednesday's shooting revived memories of another shooting rampage in Fort Hood over four years ago. On Nov. 5, 2009, Army psychiatrist Nidal Malik Hasan, opened fire at a soldier readiness center on the base. The shooting left 13 people dead and more than 30 others wounded. It was the deadliest attack on a domestic military installation in U.S. history. Hasan is now on death row at the military prison at Fort Leavenworth in Kansas.