Eddie Ray Routh has begun a life sentence in prison without the possibility of parole after a Texas jury found him guilty of murdering Chris Kyle, the former US Navy SEAL whose autobiography was turned into the hit movie American Sniper.

The ex-Marine charged with shooting American Sniper author Chris Kyle and another man was not legally insane, a prosecution expert said, suggesting the man may have gotten some of his ideas from the television show Seinfeld.

Prosecutors rested their case in the trial of a former US Marine charged with killing American Sniper author Chris Kyle and another man.

Chris Kyle, who was responsible for 160 killings during his career as US Navy Seal sniper, was shot and killed alongside another man at a gun range by a former marine.