Chris Kyle, who wrote American Sniper: The Autobiography of the Most Lethal Sniper in US Military History, and his friend Chad Littlefield were shot dead at a firing range in Glen Rose, Texas, in the US, on Saturday, authorities said.

The two men are believed to have taken the suspected gunman, former US Marine Eddie Routh, 25, to the range where the shooting took place.

Captain Jason Upshaw of the Erath County Sheriff's Office said on Sunday that Kyle and Littlefield died of gunshot wounds and that Routh had been charged with two counts of murder and one count of capital murder.

"We lost two American heroes," Upshaw told reporters, noting that the weapon thought to have been used in the incident, a semi-automatic handgun, had been found at Routh's home.

Kyle, 38, was credited with more than 150 confirmed kills during a decorated decade-long service career that included four tours in Iraq. Since leaving the SEALs, he had helped run a support group for struggling former military personnel.

His memoir recounted battle experiences in Iraq and he wrote that al-Qaeda militants whose comrades he had gunned down dubbed him "The Devil" and said they had put a bounty on his head.

Sheriff Tommy Bryant said Routh was believed to be suffering from post-traumatic stress disorder and that his mother may have contacted the veterans support foundation that Kyle was involved with.

Kyle's death had earlier been confirmed by Fitco Cares, a support group he helped start which worked with returning soldiers who had the disorder.

"My heart is breaking," said Fitco's director, Travis Cox, noting that the former sniper leaves a wife and two children.

"Chris died doing what he filled his heart with passion - serving soldiers struggling with post-traumatic stress disorder."