Spc. Ivan Lopez, 34, was on medication "for depression and anxiety and a variety of other psychological and psychiatric issues," and was being evaluated for possible post-traumatic stress disorder, according to Fort Hood commander Lt. Gen. Mark Milley.
"He was not diagnosed, as of today, with PTSD. He was undergoing a diagnosis process to determine if he had PTSD. That is a lengthy process," Milley said.
Lopez had served four months of combat duty in Iraq in 2011, but he saw no combat and was never wounded in action, according to military records.
The soldier saw a psychiatrist last month and showed no "sign of any likely violence either to himself or others," Army Secretary John McHugh said. His record shows "no involvement with extremist organizations of any kind."
While there was no indication that the incident was related to terrorism, investigators said nothing is being ruled out just yet. Investigators spoke with the man's wife and searched his home on Thursday.
Authorities are also looking into whether an argument or fight may have triggered the attack.
Lopez apparently walked into a building and began firing a .45-caliber semi-automatic pistol on Wednesday afternoon. He then got into a vehicle and continued firing before entering another building and kept shooting.
Milley said Lopez was met by a military police officer in a parking lot on the base. The shooter held his hands up, then reached for his gun. The female officer "engaged the shooter," at which point Lopez fatally shot himself.
His weapon had been recently purchased in the local area and was not registered to be on the base, Milley said. Lopez arrived at Fort Hood in February from another base in Texas.
The shooting happened at the same Texas military base where 13 people were killed and more than 30 wounded in a 2009 attack. Army psychiatrist Nidal Hasan was convicted and sentenced to death last year.
The Associated Press contributed to this report.