A troubled US soldier fired his pistol at least 35 times during an eight-minute killing spree at the Fort Hood military base after an argument over a request for leave, investigators say.
Army Criminal Investigation Command spokesman Chris Grey said on Monday that Specialist Ivan Lopez killed three people and wounded 16 on Wednesday.
Grey did not, however, indicate whether officials had granted Lopez's request for leave.
The shooting took place in an area equivalent to two city blocks, as Lopez proceeded to shoot from his slowly moving vehicle after the argument.
Since Wednesday's killings, investigators have managed to retrace the path followed by Lopez, 34.
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They found 235 clues on the ground and interviewed more than 1100 witnesses.
Everything started with a "verbal altercation concerning his request for leave and the processing of that request at his unit administrative office" at Building 39001 of the sprawling Texas military base, Grey said.
"Within minutes of the altercation, the subject brandished a .45 calibre semi-automatic handgun and fired multiple rounds, shooting one soldier who later died of his wounds and wounding 10 additional soldiers."
The soldier who was killed and at least one of the wounded soldiers had been involved in the argument.
Lopez then got into his own vehicle and drove very slowly northbound in the southbound lane, shooting at two soldiers and wounding one of them as he headed toward his unit's Building 40027.
He entered the building, opening fire and killing one soldier before wounding two others in the adjacent parking area.
He then returned to his vehicle, firing into the front windshield of a vehicle and wounding one of two soldiers inside before wounding another soldier walking outside the medical brigade building.
At one point, he exited his vehicle again and entered the medical building, killing a soldier on duty at the main entrance and wounding another.
"At this point, we do not know why he entered that building and we may never know why," Grey said.
Lopez then got back into his vehicle one last time, turning into a Building 39002 parking lot.
During a verbal exchange, a military police officer responding to the shootings drew her weapon and fired one round when Lopez "reportedly brandished his weapon," according to Grey, noting that an autopsy revealed that was not the shot that killed the soldier.
"The subject then allegedly placed his .45 calibre handgun to his head and took his own life."
Grey say investigators, who have yet to establish a motive, estimate that Lopez fired more than 35 rounds of .45 calibre ammunition.
Three were collected in his car. Another 32 shell casings were picked up at various locations.
President Barack Obama and his wife Michelle are due to visit Fort Hood for a ceremony to pay respect to the victims Wednesday.
