Chris Kyle, a former U.S. Navy SEAL credited with the largest number of confirmed kills, was one of two people fatally shot at a North Texas shooting range Saturday, Texas Highway Patrol confirmed to Fox News.

Eddie Ray Routh of Lancaster was arraigned early Sunday on two counts of capital murder in the deaths of Kyle, 38, and Chad Littlefield, 35, at the shooting range about 50 miles southwest of Fort Worth.

Capt. Jason Upshaw with the Erath County Sheriff's Office said Routh used a semi-automatic handgun, which authorities later found at his home. Upshaw said ballistics tests weren't complete Sunday, but authorities believe the gun was used in the shootings. Upshaw declined to give any more details about the gun.

Routh has not made any comments indicating what his motive may have been, Upshaw said. Sheriff Tommy Bryant said Routh was unemployed and "may have been suffering from some type of mental illness from being in the military himself," but he didn't know if Routh was on any medication.

"I don't know that we'll ever know. He's the only one that knows that," Upshaw said.

The U.S. military confirmed Sunday that Routh was a corporal in the Marines, serving in active duty from 2006 to 2010. He was deployed to Iraq in 2007 and Haiti in 2010. His current duty status is listed as reserve.

Routh is being held on $3 million bond. Authorities did not know whether Routh had a lawyer yet.

Kyle, a decorated veteran, wrote the best-selling book, "American Sniper: The Autobiography of the Most Lethal Sniper in U.S. Military History," detailing his 150-plus kills of insurgents from 1999 to 2009. Kyle said in his book that Iraqi insurgents had put a bounty on his head. According to promotional information from book publisher William Morrow, Kyle deployed to Iraq four times.

Bryant said the trio went to the shooting range around 3:15 p.m. Saturday. A hunting guide came across the bodies of Kyle and Littlefield around 5 p.m. and called 911.

Upshaw said autopsies were still pending and he could not say how many times the men were shot or where on their bodies they were hit.

After the shootings, Routh left the shooting range in Kyle's black pickup truck, Bryant said, first going to his sister's home in Midlothian, where he told her and her husband what he had done. The couple called local police.

Routh arrived at his home in Lancaster, about 17 miles southeast of Dallas, at about 8 p.m. Police arrested him after a brief pursuit and took him to the Lancaster Police Department.

Travis Cox, the director of a nonprofit Kyle helped found, told the Associated Press on Sunday that Kyle and Littlefield had taken Routh to the range. Littlefield was Kyle's neighbor and "workout buddy," Cox said.

"What I know is Chris and a gentleman -- great guy, I knew him well, Chad Littlefield -- took a veteran out shooting who was struggling with PTSD to try to assist him, try to help him, try to, you know, give him a helping hand and he turned the gun on both of them, killing them," Cox said.

A knock on the door at Routh's last known address went unanswered Sunday. A for-sale sign was in front of the small, wood-framed home.

Kyle's nonprofit, FITCO Cares, provides at-home fitness equipment for emotionally and physically wounded veterans.

"Chris was literally the type of guy if you were a veteran and needed help he'd help you," Cox said. "And from my understanding that's what happened here. I don't know how he came in contact with this gentleman, but I do know that it was not through the foundation."

Cox described Littlefield as a gentle, kind-hearted man who often called or emailed him with ideas for events or fundraisers to help veterans. He said he was married and had children.

"It was just two great guys with Chad and Chris trying to help out a veteran in need and making time out of their day to help him. And to give him a hand. And unfortunately this thing happened," Cox said.

Bryant seemed to confirm that scenario. The sheriff said Routh's mother "may have reached out to Mr. Kyle to try to help her son."

"We kind of have an idea that maybe that's why they were at the range for some type of therapy that Mr. Kyle assists people with. And I don't know if it's called shooting therapy, I don't have any idea," Bryant said.

Lt. Cmdr. Rorke Denver, who served with Kyle on SEAL Team 3 in Iraq in 2006, called Kyle a champion of the modern battlefield. Denver wasn't surprised that Kyle apparently used a shooting range to help someone with PTSD.

"For us, for warriors, that's a skill set that has become very familiar, very comfortable for us," said Denver, a lieutenant commander in a reserve SEAL team. His book "Damn Few," about training SEALs, will be released this month. "So I actually see it as kind of a perfect use of Chris' unique skill set and expertise of which he has very few peers."

Craft International, Kyle's security training company, had scheduled a $2,950-per-person civilian training event at Rough Creek Lodge called the "Rough Creek Shoot Out!" for March 1-3. The price included lodging, meals and shooting instruction. Kyle was scheduled to teach the first class, called "precision rifle."

Kyle is survived by his wife, Taya, and their two children, Cox said.

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The Associated Press and Fox News' Jennifer Griffin contributed to this report.

