A Texas jury has found Eddie Ray Routh guilty of capital murder in the 2013 shooting deaths of former Navy SEAL Chris Kyle and his friend Chad Littlefield.

Prosecutors did not seek the death penalty, and Routh was sentenced to life in prison without the possibility of parole.

Routh admitted to killing the two men at a gun range on Feb. 2, 2013, but pleaded not guilty by reason of insanity.

Routh's attorneys said the former Marine was in the grip of a medically diagnosed psychosis at the time of the killings.

FoxNews.com reported:

In front of a packed courtroom, Erath County assistant District Attorney Jane Starnes and three defense attorneys made their case.

"That is not insanity. That is just cold, calculated capital murder," Starnes said. "(Routh) is guilty of capital murder and he was not by any means insane."

But defense attorneys contended that Routh could not have realized what he was doing on Feb. 2, 2013.

"He didn't kill those men because of who he wanted to be, he killed those men because he had a delusion," Warren St. John said. "He thought that they were going to kill him."

At one point, Taya Kyle, Chris Kyle's widow, stormed out of the courtroom in the middle of the defense's closing arguments, whispering an expletive and slamming her hand on the wall as she walked out the door. At the time, attorneys were discussing how useful it would have been for Routh's mother to have told Chris Kyle about her son's history of violence.

Kyle and Littlefield took Routh, who had deployed to Iraq and earthquake-ravaged Haiti, to a shooting range after Routh's mother asked Kyle to help her son cope with PTSD and other personal demons. Interest in the trial had been partially driven by the Oscar-nominated film based on Kyle's life.

Routh's attorneys also pointed to the gunman's use of Kyle's pickup truck after the shooting to purchase tacos at a drive-through window and run assorted errands as evidence of delusional behavior.

Had Routh been found not guilty by reason of insanity, the state could have moved to have him committed.

Routh's attorneys pointed out that they needed only a preponderance of evidence for jurors to conclude Routh was insane at the time of the shootings and therefore not guilty, a standard of proof well below what would be required to convict him of capital murder.

But prosecutors also noted that Routh had apologized to Kyle's family -- evidence, they said, of a guilty mind.

"This defendant gunned down two men in cold blood, in the back, in our county. Find him guilty," Erath County District Attorney Alan Nash said.

Watch the verdict being read in the clip above.