A former Marine was found guilty late Tuesday of the 2013 shooting deaths of former Navy SEAL Chris Kyle, the author of "American Sniper," and his friend Chad Littlefield.

It took an Erath County, Texas jury less than two hours to convict Eddie Ray Routh of capital murder. State District Judge Jason Cashon sentenced Routh to life in prison without the possibility of parole. Prosecutors had not sought the death penalty in the case. Routh's defense team said they would appeal the conviction.

"We have waited two years for God to get justice on behalf of our son," Littlefield's mother, Judy, told reporters outside the courthouse. "And as always, God has proven to be faithful, and we're so thrilled that we have the verdict that we have tonight."

Chris Kyle's widow, Taya, was not in the courtroom when the verdict was read. Earlier in the day, she had 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. 

Routh showed no visible emotion as the verdict was read, while Kyle's brother and parents were among a group of the victims' families and friends who cried and held hands. They did not issue a statement.

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Jerry Richardson, Littlefield's half-brother, told Routh that he "took the lives of two heroes, men who tried to be a friend to you, and you became an American disgrace." Routh had no reaction.

Texas Gov. Greg Abbott tweeted "JUSTICE!" in response to the verdict.

Routh, 27, had admitted to killing Kyle and Littlefield at a gun range on Feb. 2, 2013 but pleaded not guilty. His attorneys and family members asserted that he suffers from psychotic episodes caused by post-traumatic stress disorder and other factors.

But prosecutors said Tuesday that whatever episodes Routh suffers are self-induced through alcohol and marijuana abuse.

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.

"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."

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 blockbuster 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.

Kyle made more than 300 kills as a sniper for SEAL Team 3, according to his own count. After leaving the military, he volunteered with veterans facing mental health problems, often taking them shooting.

Fox News' Jennifer Girdon and The Associated Press contributed to this report.