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, also a veteran, were gunned down Saturday afternoon on 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.