Investigators said Friday they may never know exactly what drove a troubled U.S. soldier to shoot dead three people and wound 16 earlier this week at Fort Hood.
Army specialist Ivan Lopez, 34, went on the rampage on Wednesday at the sprawling Texas military base before turning the .45-caliber semi-automatic pistol on himself.
On Thursday his base commander said Lopez may have argued with one or more colleagues before the shooting, and Lieutenant General Mark Milley also said that Lopez's troubled mental state was likely "the fundamental underlying causal factor."
Chris Gray, spokesman for the CID, the Army's criminal investigation service, said Friday: "At this time, we have not established a concrete motive but we will do everything in our power to do so.
"Given that the alleged shooter is deceased, the possibility does exist that we may never know exactly why the alleged shooter did what he did."
Some 150 investigators including FBI agents are attempting to explain what triggered Lopez's armed assault. They have heard from more than 900 witnesses.
Lopez's four-month deployment in Iraq in 2011 are of particular interest.
Milley said: "We are digging into his combat experience in Iraq. And so far, we have not discovered any specific traumatic event, wounds received in action, contact with the enemy or anything else specific that he may have been exposed to while deployed."
They do not think that Lopez's mental state was "the direct precipitating factor," said Milley, adding that it is not thought that Lopez singled out any of his victims.
"We believe it was more likely an escalating argument in his unit area. But we're still conducting that investigation and we'll address every single one of the causal and contributing factors that resulted in this horrible tragedy."
Ten of the 16 wounded have been released from hospital.
[Image: "U.S. Army Uniform" on Shutterstock]