An Iraq war veteran was facing murder charges Monday for allegedly gunning down two men, one of whom was a former Navy SEAL sniper whose exploits in the same conflict were detailed in a best-selling book.

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

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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.

A member of SEAL Team 3, Kyle he provided protection for Marines and other US troops in combat zones. His longest and most remarkable kill – from 1.2 miles (two kilometers) away – took out an insurgent aiming a rocket launcher at an approaching Army convoy.

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Since leaving the SEALs, he had helped run a support group for struggling ex-military personnel.

His memoir recounted battle experiences in the Iraqi rebel strongholds of Ramadi and Fallujah, 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 “some type of mental illness,” or post-traumatic stress disorder (PTSD) and that his mother may have contacted the veterans support foundation that Kyle was involved with.

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Kyle’s death had earlier been confirmed by FITCO Cares, a support group he helped start which worked with returning soldiers who had PTSD.

“My heart is breaking,” said FITCO’s director, Travis Cox, noting that the former sniper leaves a wife and two children.

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“Chris died doing what he filled his heart with passion — serving soldiers struggling with the fight to overcome PTSD.”

The US military confirmed on Sunday that Routh had seen active service with the Marines in Iraq, but that he was currently listed as a reserve.

The Dallas Morning News wrote that Kyle was awarded two Silver Stars and five Bronze Stars with Valor for his military service.

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The fatal shooting comes amid a raging and bitterly fought debate in the United States over gun control, after several mass shootings in which publicly available high-powered, military-style weapons have been used.

The issue of gun killings at the hands of people suffering from mental or emotional illness has figured prominently in such arguments, with calls for tighter background checks for gun owners.

In December, a mentally disturbed man killed 20 young children and six adults at the Sandy Hook elementary school in Newtown, Connecticut in a massacre that shocked the nation.

President Barack Obama has since put gun control on his legislative agenda with plans to ban assault rifles and high capacity magazines at its forefront, and on Monday he will travel to Minneapolis to begin a fresh push on the issue.

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The measures, however, are up against fierce opposition from gun advocates, most notably the National Rifle Association, given that the right to bear arms is enshrined in the US constitution.