Caleb Barnes In this May 14, 2015, file photo, Army Spc. Caleb Barnes, center, is escorted to court for a preliminary hearing in Allentown, Pa. As a murder trial gets underway for Army Spc. Caleb Barnes, accused of killing his then-14-year-old girlfriend's mother after the woman objected to their age difference, his attorney Richard Webster told a jury Tuesday, Aug. 9, 2016, that Cheryl Silvonek was fatally stabbed by her daughter Jamie Silvonek in March 2015.

ALLENTOWN, Pa. — A soldier accused of killing his then-14-year-old girlfriend's mother because she disapproved of their relationship has been found guilty of first degree murder.

It took the jury about four hours to reach its verdict Friday in the trial of Army Spc. Caleb Barnes, The Morning Call of Allentown reported. First degree murder carries a mandatory life sentence. Barnes will be sentenced on Sept. 19.

During his testimony Friday, Barnes placed the blame squarely on the teen Friday, telling jurors that she committed the murder and his only role was helping dispose of evidence because she claimed to be pregnant.

His testimony came after Jamie Silvonek, now 15, took the stand for the defense but told a wildly different story that implicated him in the slaying.

Barnes, who's from El Paso, Texas, is accused in the stabbing death of Cheryl Silvonek in March 2015 in Pennsylvania because she disapproved of his relationship with her daughter because of their age difference. Barnes, now 22, was 21 at the time of the killing.

Jamie Silvonek had agreed to testify against Barnes as part of a plea deal, but prosecutors rested their case Thursday without calling her. Silvonek, who was tried as an adult, pleaded guilty in February to first-degree murder and criminal conspiracy and is serving 35 years to life.

On Friday, in a twist, the defense called her to the stand, where she told the court she was "inexorably, ineffably in love" with Barnes, The Morning Call reported.

The teen admitted urging Barnes via text to carry out the slaying so they could continue their relationship.

"I believe what I did was just as bad or worse than the physical act," she said. "I realized that I had to do the right thing. That's taking accountability for my actions, something your client has yet to do."

Silvonek described Barnes reaching for her mother's throat from the back seat of her SUV as the woman parked in the driveway of her home after driving the couple to a concert in Scranton.

Silvonek, wearing a beige jumpsuit and shackles, wiped away tears as she talked about her mother, her voice cracking with emotion. She stared at her lap, her long blond hair covering her face.

Barnes, in a suit and tie, glared at Silvonek while she testified, prompting a reprimand from the judge.

"You will not have that kind of eye contact with this witness. Are we clear?" Judge Maria L. Dantos told Barnes.

Barnes later told the court that Silvonek stabbed her mother to death during a fight after she revealed she was pregnant. Senior Deputy District Attorney Jeff Dimmig called Barnes' account "nonsense" that slandered the girl.

Barnes said he had fallen asleep in his Camaro parked outside the Silvoneks' house before heading back to Fort Meade in Maryland, where he was based. He was off-duty at the time of the killing.

He said he was woken up by Jamie Silvonek, covered in blood, banging on the car window. She told him her mother was dead in the SUV, Barnes testified.

Dimmig showed Barnes the murder weapon. Barnes told the prosecutor it was a knife he had left at the Silvoneks' home the week before the killing and he had never gotten it back from his girlfriend, according to Lehigh Valley Live.

"You didn't pick this out special?" Dimmig asked.

"That's a utility knife. That's not a knife for killing someone," Barnes answered.

The jury began deliberations late Friday afternoon.