Military investigators say the Fort Hood soldier who shot and killed three people, and injured 16 others, was involved in a verbal altercation with soldiers shortly before the attack on the Texas army base.
Chris Grey, a spokesman for the U.S. Army Criminal Investigation Command, says 34-year old Army specialist Ivan Lopez may have argued with soldiers from his unit shortly before opening fire.
Grey says, however, that a "concrete motive" has not been established.
Authorities say Lopez walked into a base building Wednesday afternoon and began firing a semi-automatic pistol. He then got in a vehicle and continued shooting before entering another building. Officials say Lopez shot himself in the head after being confronted by military police.
Base commander Lt. General Mark Milley has said Lopez had a history of psychological conditions which may have been an underlying cause for the attack.
Authorities say he was taking medication for depression, anxiety and sleep disturbances and had asked for an evaluation for post traumatic stress disorder, a mental affliction usually associated with the extreme stress or danger soldiers encounter in combat.
Lopez served for four months in Iraq in 2011, but did not see any combat.
Investigators say they have interviewed more than 900 people to gather details of the crime scene.
Wednesday's attack was the second at the base since 2009 when an Army psychiatrist opened fire on fellow soldiers, killing 13.