A marine has been found guilty of murdering famed US Navy SEAL Chris Kyle at a gun range two years ago.

Eddie Ray Routh, 27, was convicted by a court in Stephenville, Texas for shooting Kyle and Chad Littlefield multiple times in February 2013, then fleeing in Kyle's pickup truck.

Jurors decided not to declare him not guilty by reason of insanity. He will serve life in prison without possibility of parole, after prosecutors decided not to pursue the death penalty.

The jury, which reached its decision after just two and a half hours, was made up of ten women and two men. Their verdict was unanimous.

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Before the jury retired to consider its verdict, crime scene analyst Ryan Howard said Kyle's gun wounds were close together, suggesting he had been hit in the spinal cord and had not moved much when he was struck.

"He absolutely never saw this coming," he told the court.

Last week, a psychiatrist testified that Routh is a paranoid schizophrenic and showed signs of psychosis that could not be faked.

The jury heard how Routh gunned down the men after believing they - and other people around him, including his colleagues and girlfriend - were human-pig hybrids. If he did not kill them, they were going to kill him, he told the psychiatrist.

Routh's friends and family have also testified that his behavior in the weeks before the shootings was increasingly erratic and that 'he did not seem all there' after gunning down the two men.

They said he was unable to care for himself, acted as if he believed that someone was going to hurt him and thought that the government was listening to him.

"It was clear that he was psychotic," Mitchell Dunn, a forensic psychiatrist for the defence, told the court on Thursday. "There was no question about that."

Seinfeld defence

But prosecutors contended Routh knew what he was doing and acted deliberately when he shot the two and then fled.

They called a psychologist who testified Routh has a personality disorder made worse by heavy drug use and has been faking schizophrenia, calling his condition "cannabis-induced psychosis".

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Routh's uncle had previously testified that they shared some potent marijuana and perhaps consumed whiskey on the morning of the shooting.

"In my opinion, he did know what he was doing was wrong, and he did it anyway," said Dr Randall Price, the forensic psychologist.

On Friday, Dr Price repeatedly accused Routh of "setting the stage" for an insanity defense and suggested Routh may have gotten the idea from Seinfeld or the TV show Boss Hog.

In a Seinfeld episode, one character, Kramer, believes he has discovered a "pig-man".

"I don't know for a fact that he saw that episode of Seinfeld but I do know that he's watched Boss Hog," Dr Price said.

"For a long time, he's talked a lot about pigs to a lot of people. So it's suspicious."

He and another prosecution expert, Dr Michael Arambula, said Routh's actions did not meet the legal definition of insanity under state criminal law.

"Any time intoxication is present, the game is over," Dr Arambula testified.

The trial has focused renewed attention on Kyle, who is credited with the most confirmed kills of an US military sniper, and on the movie American Sniper, which was based on his memoir.

Routh, who served with the Marines in Iraq and Haiti, had been admitted to Veterans Affairs hospitals on multiple occasions after his service to treat mental illness.

Prosecutors said Routh did not see combat in his overseas deployments.

Routh's attorneys never disputed that he shot Kyle and Littlefield, and instead rested their case on whether he was in a psychotic state at the time of the killing.

In videotape provided by police and shown in court in the rural Texas city of Stephenville, Routh admits to the shooting in a rambling speech that defense attorneys contend attests to his unstable mental state.

- Daily Mail