Taya Kyle, wife of Chris Kyle, with Don and Judy Littlefield, parents of Chad Littlefield, during the capital murder trial of Eddie Ray Routh at the Erath County, Donald R. Jones Justice Centre in Stephenville, Texas. Photo: Michael Ainsworth, The Dallas Morning News

Stephenville, Texas - A jury in Texas on Tuesday found Eddie Ray Routh guilty of murdering Chris Kyle, the former US Navy SEAL whose autobiography was turned into the hit movie American Sniper.

Routh, 27, was sentenced to life in prison without the possibility of parole for fatally shooting Kyle and Kyle's friend, Chad Littlefield, multiple times at a gun range about 110km south-west of Fort Worth in February 2013.

In closing arguments before the case went to the jury, prosecutor Jane Starnes said Eddie Ray Routh, 27, waited for the right time before fatally shooting Kyle and Kyle's friend Chad Littlefield at the range about 110km south-west of Fort Worth in February 2013.

“The defendant did the murder. How much clearer can it be? This is not a whodunit case,” she told jurors in the rural Texas city of Stephenville. Prosecutors presented video and audio evidence where Routh admitted to the crime.

Defence lawyers argued that Routh, a former US Marine, was a paranoid schizophrenic and should be declared innocent by reason of insanity. They have not refuted charges Routh shot the two and fled in Kyle's pickup truck.

“That is not insanity. That is just cold, calculated capital murder,” Starnes said. Prosecutors are seeking a life sentence without the possibility of parole.

Defence lawyers told the jury that Routh had been to VA hospitals four times because of his mental illness and was diagnosed as psychotic. They said he was suffering a paranoid episode when he went to the range and met the state's legal definition of insanity.

“He killed those men because he had a delusion. He believed in his mind that they were going to kill him.” Warren St John said in closing arguments.

During the defence arguments, Kyle's widow left the courtroom, slamming the door behind her.

Jurors began deliberations after the arguments ended.

The judge told jurors they could find a person innocent by reason of insanity if the defendant did not know the conduct was wrong due to a severe mental defect or illness.

A forensics expert called by prosecutors said Kyle and Littlefield were shot in the back at close range. They had no time to remove loaded guns that they had holstered.

“He (Kyle) absolutely never saw this coming,” said crime scene analyst Howard Ryan.

Prosecutors said the two were shot by 12 or 13 bullets and that Routh put on act to get out of trouble.

“He is capable of dreaming up excuses to get his hide out of trouble at convenient times depending on who he is talking to,” prosecutor Alan Nash said. Prosecutors said Routh waited until Kyle's gun was empty at the range before opening fire.

The trial has focused renewed attention on Kyle, credited with the most confirmed kills of any US military sniper, and how he tried to help fellow veterans manage their mental scars by taking them for outings at gun ranges, sometimes with Littlefield.

Defence attorneys recalled a psychiatrist to the stand on Tuesday who said Routh had psychotic bouts of paranoia and suffered from schizophrenia at the time of the incident, believing he had to kill the two before they killed him first.

“It is my opinion that it is not a mood disorder that Mr Routh suffered from,” said Dr Mitchell Dunn.

Prosecutors in a trial that has seen nine days of testimony, had called a psychologist who testified Routh had a mood disorder made worse by heavy drug use and had been faking schizophrenia.

Routh served with the Marines in Iraq and Haiti. Prosecutors said Routh did not see combat in those deployments.