Chris Kyle, a former US Navy SEAL sniper who was responsible for 160 kills during his career, has been shot dead at a gun range, local media reports.
A suspect, identified as Eddie Ray Routh, was arrested, according to reports.
Kyle, who wrote American Sniper about his military service from 1999 to 2009, and another man were found dead at the Rough Creek Lodge's shooting range on Saturday, according to the Fort Worth Star-Telegram, quoting Erath County Sheriff Tommy Bryant.
The second victim was not identified.
"It just comes as a shock and it's staggering to think that after all Chris has been through, that this is how he meets his end, because there are so many ways he could have been killed" in Iraq, Scott McEwen, who co-authored Kyle's book, said.
Rough Creek Lodge is located in Glen Rose, Texas, which is about 80km southwest of Fort Worth.
Kyle, who is said to be the deadliest sniper in US military history, served four combat tours of duty in Operation Iraqi Freedom and elsewhere, and won two Silver Stars and five Bronze Stars for bravery, according to his book.
After leaving the Navy, Kyle founded Craft International, a firm that provided combat and weapons training to military, police, corporate and civilian clients.
In a 2012 interview with Time magazine he was asked what went through his mind when he aimed at a target.
"The first time, you're not even sure you can do it," he said. "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 IED. You don't think twice about it."
Kyle is the co-author of another book coming out in May under the title American Gun - A History of the US in Ten Firearms"
For the book he fired 10 weapons - such as the Spencer repeating rifle from the Civil War, the Colt .45 used in the Old West and the Thompson sub-machine gun popular with law enforcement and gangsters in the 1920s - and discussed their parts in US history.
In the wake of the slayings of 20 children and six adults at a school in Newtown, Connecticut in December, Kyle was interviewed about rising calls for curbing gun violence in the United States.
He told the website guns.com that he favoured arming teachers who have been screened and trained and spoke against restrictions on gun owners.