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, allegedly at the hands of a fellow military veteran, officials say.

Chris Kyle, the author of the best-selling "American Sniper," and Chad Littlefield, Kyle's friend, were gunned down Saturday afternoonon the grounds of the expansive Rough Creek Lodge and Resort in Glen Rose, Texas, southwest of Fort Worth, law enforcement officials said.

About four hours afterward, and 90 miles from where those two men's bodies were found by a hunting guide, authorities arrested suspect Eddie Ray Routh, 25, on a capital murder warrant.

Routh's family members could not be reached immediately for comment Sunday. No attorney has made a public statement on his behalf.

Erath County Sheriff Tommy Bryant said Sunday that he believes the suspect is in the process of seeking a court-appointed attorney.

While Routh's sister said he had admitted shooting Kyle and Littlefield, he didn't explain to her why he did it, Bryant said. Routh isn't believed to have confessed to local authorities about killing the two men. The motive for the killings is unclear.

"I don't know that we'll ever know," Erath County Sheriff's Capt. Jason Upshaw said Sunday. "(Routh) is the only one that knows that."

Sheriff: Suspect spent four years in Marines

The Rough Creek Lodge is a large facility that draws couples getting married, business people using its conference center and families looking for a getaway. Hunting and shooting sports are some of the many recreational options available on its grounds.

Kyle, Littlefield and Routh were three such visitors, arriving together around 3:15 p.m. (4:15 p.m. ET) Saturday and proceeding to a shooting range within the resort's 11,000 acres, Bryant told reporters Sunday. The range is in a "very remote part" of the sprawling complex, Upshaw explained.

"So there wasn't anybody anywhere close to that," he said, explaining there are no known witnesses.

The first sign something was wrong came when a hunting guide tied to the facility found 38-year-old Kyle and 35-year-old Littlefield -- both unconscious -- around 5 p.m., Bryant said.

He went to the lodge and called 911. Law enforcement officers followed up and found the bodies but no sign of Routh. He had taken off in Kyle's black Ford pickup, the Erath County sheriff said.

Around the time an all-points bulletin went out for that truck, authorities got a call from Routh's sister, who reported that her brother had driven about 65 miles to her home in Midlothian, Texas, and admitted to shooting Kyle and Littlefield.

Routh left his sister's house, and police eventually caught up with him -- and the truck -- shortly before 8 p.m. at his home in Lancaster, a Dallas suburb some 90 miles northeast of the shooting scene. While talking with authorities outside, he'd somehow gotten back in the truck and sped away.

Authorities chased him and finally stopped him, around 9 p.m., after spiking his tires some four to six miles down the road, according to Bryant.

Routh did not struggle with officers as they were arresting him, Bryant said. On Sunday morning, he was arraigned on murder charges and ordered held on $3 million bond.

After that, Routh sat in a jail cell "all by himself," separated from the rest of the inmate population and watched closely by guards, according to Bryant.

So who is Eddie Ray Routh?

The sheriff said that Routh spent four years in the Marines. He is believed to have left the service in 2010, according to a U.S. military official who spoke on condition of anonymity because of the sensitivity of the situation. The official did not have information on where Routh served or whether he took part in combat.

Public records show Routh previously lived at Camp Lejeune in North Carolina, though his latest address was in Lancaster, Texas.

At the time of shooting, he was unemployed. Bryant said that Routh's mother, a longtime schoolteacher, "may have reached out to Mr. Kyle to try and help her son."

'He led by example,' Kyle's friend says

Kyle learned to shoot on hunting trips with his father, then went on to serve four combat tours in Iraq with the SEALs, though his official biography notes he also worked with Army and Marine units. He received two Silver Stars and other commendations before leaving the Navy in 2009 -- claiming that, in his years as a sniper, he'd killed more than 150 people, which he called a record for an American.

In the interview with Time magazine (like CNN, a part of Time Warner), Kyle said he did not regret any of his kills. He also said he was "comfortable" with the possibility that that part of his life might be over.

He added, "I'm a better husband and father than I was a killer."

After having worked under the radar for so many years, he became a celebrity with the 2012 release of his book, which became a New York Times best-seller.

Defending his decision to divulge so much detail despite the secretive nature of the SEAL world, Kyle told Time that he was "not trying to glory myself."

"I didn't want to put the number of kills I had in there," he said. "I wanted to get it out about the sacrifices military families have to make."

He said that while killing did not come easy at first, he knew it meant saving lives.

"The first time, you're not even sure you can do it," he said in the interview. "But I'm not over there looking at these people as people. I'm not wondering if he has a family. I'm just trying to keep my guys safe. Every time I kill someone, he can't plant an (improvised explosive device). You don't think twice about it."

At one point, Kyle wrote, he shot a woman who was carrying a grenade while with her toddler. But he did not kill a child in Baghdad's Sadr City area who had a rocket-propelled grenade launcher. "According to the rules of engagement at the time, you could kill anyone with an RPG on sight. That day I just couldn't kill the kid. He'll probably grow up and fight us, but I just didn't want to do it.

He said the American public lives "in a dream world . You have no idea what goes on on the other side of the world. The harsh realities that these people are doing to themselves and then to our guys. And there are certain things that need to be done to take care of them."

After leaving the military, he founded Craft International , a military training company. Kyle also spoke up on current events, including accusing President Barack Obama of being "against the Second Amendment" because of his gun control initiatives, according to a video interview with guns.com.

The married father of two children established the nonprofit Fitco Cares Foundation to help veterans battling PTSD -- post-traumatic stress disorder -- get access to exercise equipment.

Littlefield, who leaves behind a wife and children, was a friend who worked to help people with PTSD, said Fitco Director Travis Cox.

In a statement, the foundation described Kyle as an "American hero" and pledged to carry on his mission.

"What began as a plea for help from Chris looking for in-home fitness equipment for his brothers- and sisters-in-arms" struggling with PTSD turned into an organization that will continue after his death," Cox said in a statement.

"Chris died doing what he filled his heart with passion -- serving soldiers struggling with the fight to overcome PTSD. His service, life and premature death will never be in vain. May God watch over his family and all those who considered Chris a true friend."

His friend, Jason Kos, offered similarly glowing sentiments, telling CNN's "Early Start Weekend" that Kyle was "a man of incredible character."

"He led by example," Kos said. "He always stopped to take time to talk to whoever was around him. Just incredibly humble, very funny as well."

Correction: Earlier versions of this report incorrectly described the military background of Chad Littlefield, who was killed with Chris Kyle. Littlefield was not a veteran.