By David DeKok

ALLENTOWN, Pa. (Reuters) - A 14-year-old girl and her soldier boyfriend spent time on the phone plotting murder a week before her mother, who had tried to break up their relationship, was found with her throat slashed, the girl's best friend testified on Thursday.

Jamie Silvonek, an eighth grader with braces on her teeth, sobbed as her friend, identified only as Witness 1, testified at a preliminary hearing for Silvonek and boyfriend Caleb Barnes, 21, a soldier at Fort Meade, Maryland. They are accused of killing Cheryl Silvonek on March 15 because she tried to end their relationship after finding out they were having sex.

The gruesome details of the slaying in the driveway of the family home in Oreland, Pennsylvania, unfolded at Lehigh County Courthouse in Allentown.

Witness 1 testified about what she heard as Silvonek and Barnes plotted the killing in a March 7 conversation over a cell phone in speaker mode.

"She would have her parents go somewhere, Caleb would kill them, and they would dispose of the bodies together," Witness 1 Said.

About a week later, Silvonek's mother's slashed body was found in a shallow grave a mile from the home.

Police said Silvonek told them her mother had driven the young lovers to a rock concert in Scranton and at some point on the way, made a stop. When the mother returned to the vehicle she found the couple having sex, police said Silvonek told them.

During the drive, the couple sat in the back seat and Silvonek sent multiple text messages to Barnes urging him to kill her mother, said prosecutor Jeffrey Dimmig.

Returning home from the concert, the mother parked on the home's driveway but from there the details of her killing differ according to the suspects' confessions to police, Detective Richard Heffelfinger of the Lehigh County District Attorney’s Office testified in court.

"She had bruises on her forehead, both eyes, on her mouth, three gaping wounds to her throat, and defensive wounds on her hands," Heffelfinger testified.

Silvonek recalled that Barnes grabbed her mother in a chokehold from behind, Heffelfinger said. She kicked the horn several times and scratched Barnes until he pulled out a knife and slashed her throat three times, Heffelfinger said.

Barnes told police that the mother had gone into the house and when she returned, she found the couple having sex in the vehicle, Heffelfinger testified Barnes told him. The mother attacked Silvonek and then him, and he slashed her to defend himself, Heffelfinger said Barnes told him.

After finding the mother's body, Heffelfinger said, police went to the Silvonek home, and found the couple in bed together and the bloody knife on a backyard picnic table.

(Editing by Barbara Goldberg and Bill Trott)