A Texas jury has convicted a former U.S. Marine on Tuesday of killing "American Sniper" author and Christian Chris Kyle and his friend, Chad Littlefield, at a shooting range two years ago, rejecting arguments that his mental illness played a role in their deaths.

John L. Mone and Jamie Stengle of the Associated Press reported that the trial of the former Marine, 27-year-old Eddie Ray Routh, garnered intense public attention thanks to the blockbuster Clint Eastwood film that focused on the life of the former Navy SEAL and his memoir about doing four tours in Iraq. He was automatically sentenced to life without parole for the deaths.

"We just want to say that we've waited two years for God to get justice for us on behalf of our son," Littlefield's mother, Judy, said after the verdict was handed down. "We're so thrilled that we have the verdict that we have tonight - and thank you guys for being so compassionate and treating us with respect and honoring us."

According to Lydia Warren and Kieran Corcoran of the Daily Mail, Kyle and Littlefield took Routh to a shooting range at Rough Creek Lodge and Resort on Feb. 2, 2013, upon the advice of Routh's mother, who claimed that her son was a troubled man.

"Family members say Routh suffered from post-traumatic stress disorder from serving in Iraq and in Haiti after the devastating 2010 earthquake," Warren and Corcoran wrote.

The Associated Press reported that jurors rejected defense arguments that Routh was insane and suffered from psychosis. His defense team indicated that they would appeal the conviction.

"It was clear that he was psychotic," forensic psychiatrist Mitchel H. Dunn said on behalf of Routh's defense last week. "There was no question about that."

However, the prosecution called up its own psychologist to the stand, who, according to the Daily Mail, testified that Routh faked schizophrenia and had a personality disorder made worse by heavy drug use, describing his condition as "cannabis-induced psychosis."

"The prosecution painted Routh as a troubled drug user who knew right from wrong, despite any mental illnesses," Mone and Stengle wrote. "While trial testimony and evidence often included Routh making odd statements and referring to insanity, he also confessed several times, apologized for the crimes and tried to evade police."

The Associated Press added that jurors were given three options in deciding Routh's fate: find him guilty of capital murder, find him not guilty, or find him not guilty by reason of insanity. Jurors ended up with a guilty conviction.

The Daily Mail reported that Littlefield's brother-in-law, Jerry Richardson, addressed Routh after sentencing.

"You took the lives of two heroes, men that tried to be a friend to you," Richardson said. "You became an American disgrace."

As for Kyle's family, the Associated Press reported that his wife, Taya Kyle, left the courtroom and did not return to hear the verdict. His brother and parents did not issue a statement after the verdict.