Two teenagers charged in the grisly killings of an elderly couple last year were sentenced to life in prison plus 21 years with the possibility of parole.

Cassandra Bjorge, 17, and her 19-year-old boyfriend, Johnny Rider, both pled guilty on Friday in the deaths of Randall and Wendy Bjorge, both 63. The victims -- Cassandra's grandparents -- were found with their throats slit on April 8. Police believe they had been dead for about a week before their bodies were found.

Authorities also said she and her boyfriend smoked pot and partied in the house the week after the killing, caulking the doors and windows to conceal the smell of the bodies.

"It's a pretty cold-blooded killing," said Gwinnett County Deputy Chief Assistant District Attorney Trason Cason during a court appearance last year. "It was planned. It was executed correctly, according to their plan."

Police said that the teens saw the murders as just the beginning.

"Bjorge did indicate their plan was not only to kill the grandparents but also to kill Mr. Rider’s family and then ... their plan was to kill her mother and stepfather, but there wasn’t time," Cason said.

Rider had already been charged for allegedly beating his sister with a baseball bat and pepper spraying her at a different home the night of the murders. At the time, the teens were in possession of the Bjorges' car..

"They don't look like what you think a serial killer planned to like, but that's what they planned to be," Cason said.

RELATED | Mom of teen accused of killing grandparents speaks out

Police have called this a "very unique and disturbing case on so many levels," and said they may not ever find a scenario that makes sense to a "reasonable person."

According to police records, Cassandra Bjorge had previous run-ins with violent behavior. When she was 16, she was charged with assaulting her grandmother. Police had also been called to the residence several times in the past after the teen ran away.

TIMELINE | Teens accused of killing Gwinnett grandparents

Gwinnett Superior Court Judge Debra Turner handed down the sentences.