LAFAYETTE, Colo. -- Prosecutors presented their case against a teen girl and her boyfriend who are both accused of killing the girl's mother and then leaving her body in the family car for weeks.

Lafayette police Detective Scott Robinson testified Wednesday at a preliminary hearing that 18-year-old Brian Grove said he "lost it," before he knocked his girlfriend's mother unconscious and stabbed her repeatedly.

A judge on Wednesday said there was enough evidence against the teens and bound both over for trial.

Grove and his girlfriend, 15-year-old Tess Damm, are both charged as adults in the stabbing death of Damm's mother, 52-year-old Linda Damm. Each faces a life sentence if convicted of the first-degree murder charges against them.

Robinson testified that Grove said he and Linda Damm were arguing when she told him if her daughter hadn't been born, a black guy -- referring to Grove -- wouldn't be hanging around the house. That's when Grove told Robinson he "lost it."

Linda Damm was stabbed a total of 18 times in the neck and mouth, according to an autopsy.

Robinson said Grove tried to bury the body at a cemetery and ended up leaving it in a car parked in the family garage.

Robinson testified that Tess Damm was aware of Grove's plans to kill Linda Damm, but he later admitted they weren't full-fledged plans. He added that Tess Damm used her mother's debit cards after she had been killed.

He said Tess Damm told her boyfriend to kill her mother, then called him on a cell phone to ask what was taking so long.

Prosecutors tried to show she was involved in planning the slaying. They filed the first-degree charge against Tess Damm this week. She earlier was charged with conspiracy to commit first-degree murder, being an accessory to a crime and tampering with physical evidence.

Robinson testified that Smith said Grove and Tess discussed methods for killing Tess' mom one night before going to a restaurant.

Grove suggested using a large kitchen knife, while Tess suggested combining over-the-counter medication with alcohol, he said. Then the teens left the restaurant for Damm's home.

"According to Jared, she turned to Bryan and says 'Do it.' Bryan says, 'OK.' He gives her a kiss and says, 'I love you,' and then he gets out of the car," Robinson testified.

Under questioning from Grove's attorney, Kristi Sanders, Boulder County forensics pathologist John Meyer testified that he couldn't tell from the autopsy whether one, two or three knives were used to stab Linda Damm 18 times.

Defense attorneys questioned why information from another teenager involved in the case was relied upon as much as it was. They said the investigation was sloppy and questioned why other evidence was not collected.

Two other teens were charged in Linda Damm's death.

Jared Smith, 16, has pleaded guilty to being an accessory on accusations he helped put the body in the car where it was found. He was sentenced to two years in juvenile detention and six months of parole.

Jared Guy, 18, was scheduled for trial Sept. 10 on accusations that he helped the teens try to hide the body. He has pleaded not guilty to being an accessory and evidence tampering.

Linda Damm's brother, John Schueller, said he wants to see justice for his sister.

"You can't really describe the conflict and the pain and the loss of my sister and the loss of my niece. We don't really know this Tess," Schueller. "When she came to our house she was just a little girl with the mom on the holidays. We don't know this angry young woman."