Motive still unclear in ex-Navy SEAL's murder

(CBS News) Chris Kyle was the most lethal sniper in U.S. military history, notching 255 confirmed kills over a 10-year career with the Navy SEALs. He earned two Silver Stars and five Bronze Stars for his bravery.

The war hero and best-selling author was shot to death on Saturday. Not on the battlefield -- but in his home country. He was only 38 years old.

Ex-Navy SEAL Chris Kyle murdered in Texas

Kyle survived two shootings and six improvised explosive device attacks in Iraq. Three years ago, he retired from the military, started a security training company and also a charity to help returning soldiers, including those with post-traumatic stress disorder, find their way. It was while doing just that -- aiding a fellow veteran -- that he lost his life.

Kyle was so deadly accurate in combat that insurgents in Iraq put a $20,000 bounty on his head and dubbed him "The Devil of Ramadi." But to fellow SEALs like Rorke Denver, he was known as "The Legend." Denver told CBS News, "We were aware early on in that deployment that something special, for lack of a better term, was unfolding."

Kyle spotted an insurgent aiming a rocket launcher at an Army convoy in Sadr City in 2008. He shot and killed him from 21 football fields away.

On Saturday, an American veteran did what Iraqi militants could not. Kyle was allegedly killed by an apparently troubled Marine, Eddie Ray Routh, at a shooting range south of Fort Worth.

Tommy Bryant, Erath County Sheriff, said, "This shooter is possibly one of those people that he had taken out to the range, to mentor, to visit with, to help him."

Authorities in Texas recovered a semi-automatic rifle from Routh's house they believe was used in the killings, but have been unable to determine a motive. He's been arraigned on two counts of capital murder.

Denver said, "While it sounds backwards, I actually think time on the range and using those skill sets in a controlled and safe environment could be a huge benefit to veterans."

Ruth allegedly also killed Kyle's friend, Chad Littlefield, and fled the scene in Kyle's black pickup truck before being caught.

To Kyle, being a sniper was never about racking up his kill numbers, but rather, shielding his comrades. Last year, he told CBS Dallas station KTVT about Ryan Job and Marc Lee -- two fellow SEALs he couldn't protect in battle.

Kyle recalled at the time he "just sat down, put my back up against the wall, curled my knees up to my chest, put my head in my knees, and started bawling. My only regrets are the guys I couldn't save. That's what keeps me up at night."

The war took a toll on his family too, so Kyle gave up the warrior's life in 2009 for the sake of his wife and two young children. Kyle said, "I took it as an ultimatum, either you get out, or she and my kids were going to be gone."

Kyle's wife Taya said in the same interview, "Of course, he looked at that and thought the marriage would be over. And you know what, he's probably right."

As a civilian, Kyle appeared in the NBC reality show "Stars Earn Stripes" and wrote the best-seller "American Sniper." Kyle's highly-anticipated second book was due to be released in May.

Denver said, "He's one of those guys who will be in the books. When history writes of these conflicts, he will be in the conversation as one of the greats."

For Chip Reid's full report, watch the video in the player above.