The killer of American Sniper author Chris Kyle has been convicted of murder and jailed for life in Texas.

Eddie Ray Routh, 27, was found guilty of the fatal shooting of Kyle and another man at a gun range in early 2013.

Lawyers for the former marine claimed Routh had been insane when he turned a gun on Kyle, who was trying to help him deal with post traumatic stress disorder.

Huge public interest has followed the trial because Kyle's book about his exploits as the deadliest sniper in the US military has become a smash-hit film, directed by Clint Eastwood.

A court in Stephenville, Texas, was told Kyle sent a text message describing Routh as "straight up nuts" shortly before he died. Routh was also on anti-psychotic medication at the time.

Kyle and fellow victim Chad Littlefield were shot in the back more than 10 times.

Speaking in court, Littlewood's brother-in-law Jerry Richardson said: "You became an American disgrace.

"You took the lives of two heroes, men that tried to be a friend to you."