In the concluding moments of Clint Eastwood's recording breaking movie American Sniper, audiences got a blink-and-you'll-miss-it look at the man who would go on to shoot and kill the deadliest marksman in U.S military history. The actor who plays Eddie Ray Routh gets little screen time, though the man himself is set hit national headlines.

Bradley CooperBradley Cooper plays Chris Kyle in American Sniper

Routh's trial begins in a courthouse in north Texas this week. The 27-year-old does not deny killing Kyle at an upscale gun range in February 2013 though his lawyers are mounting an insanity defence, claiming Routh was was so mentally unstable he cannot be held responsible for the killings.

A jury of 12 men and women from the surrounding Erath County will decide whether Routh spends the rest of his life in Texas state prison, or a mental home.

More: why Chris Kyle's wife faces $1.4 million bill

Kyle  - played by Bradley Cooper in Eastwood's Oscar nominated movie - left the military in 2009 after tours of Iraq. He is believed to have killed 150 people.

On the day of his killing, Kyle had agreed to drive Routh to the gun range to help with his mental state - he had been struggling to get his life back together after serving in Iraq. According to a police report, Kyle's body was found at the range at around 4.50pm, lying next to the body of his friend Chad Littlefield.

"Both victims were obviously deceased, having suffered multiple gunshot wounds," the report read.

800 potential jurors will be whittled down to just 12 men and women for the trial, based on their knowledge of the shootings. The film could also be an issue and the trial will almost certainly continue during the Oscars ceremony on February 22, where it has an outside chance of winning Best Picture. 

More: Is American Sniper's Chris Kyle really a coward?