UNDATED (CNN) -- A former Navy SEAL known for claiming a record number of sniper killings in Iraq was one of two men shot dead at a Texas gun range over the weekend. He and another military veteran were allegedly killed at the hands of a fellow veteran.

Former Navy SEAL Chris Kyle, who claimed a record 150 kills in Iraq, died at the hands of another vet, authorities say, allegedly using a semi-automatic handgun. What police don't know yet is why. Capt. Jason Upshaw with the Erath Co. Sheriff's Office says, "I don't know that we'll ever know. I mean, he's the only one that knows that. But at this point, he hasn't made any comments to law enforcement as to why he did it."

Kyle, friend Chad Littlefield, and the suspect Eddie Ray Routh all drove to the gun range together in a pickup truck authorities say. Less than two hours later, police say Kyle and Littlefield were dead, Routh had fled to his sister's house. He told her what happened, she called police and Routh was eventually caught.

Routh is an Iraq war veteran who left the Marines in 2010. Kyle, who wrote the best-selling autobiography "American Sniper" talked about his duty to those he served with. In an interview with KTVT last year, Kyle said, "My only regrets are the guys I couldn't save. That's what keeps me up at night. But every shot I took, I felt extremely justified."

When he left the military, Kyle became a fierce advocate for vets, starting a foundation to help those suffering from PTSD. Authorities say that may be why Kyle and Littlefield went to the gn range with Routh. Sheriff Tommy Bryant says, "Mr. Kyle works with people that are suffering from some issues that have been in the military and this shooter is possibly one of those people that he had taken out to the range to mentor, to visit with, to help him."

Kyle's friend, Travis Cox, says that's just the kind of man he was. He says, "The man he was when he came home if i can describe it in any way is a 'servant-leader.' He served others. He was a humble, humble man. He had a vision to support veterans and their needs and he was fearless in that. And he gave his life doing so."