Johnny Rider and Cassandra Bjorge
Johnny Rider (L) and Cassandra Bjorge (R) face murder charges in Georgia. Gwinnett County Sheriff's Office

A 16-year-old Georgia girl and her teenage boyfriend are alleged to have brutally murdered her grandparents before throwing a drug-fuelled house party while their victims decomposed upstairs.

Cassandra Bjorge and her boyfriend Johnny Rider, 19, are alleged to have killed her grandparents, Randall and Wendy Bjorge, both 63, in their home in Lawrenceville earlier this month.

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The pair beat their victims with a hammer and a baseball bat before slitting their throats, according to report.

It has been reported that their bodies had laid dead for over a week in the property before they were discovered on 8 April, after reports of an assault at the address.

Police say that the pair of teenagers held a house party at the property while the bodies began to decompose, and that the duo even sealed doors and windows to contain the stench.

Bjorge and Rider are now charged with murder and aggravated assault in the deaths of the elderly couple, with Bjorge attending a preliminary hearing on Wednesday (19 April).

"This is a very unique and disturbing case on so many levels and detectives are still exploring motive," Gwinnett County police officer Deon Washington told the Gwinnett Daily Post. "We may never find a scenario that makes sense to a reasonable person."

WSB-TV reported that police said in court that the teens smoked cannabis and invited friends over after sealing two doors.

Victim duct taped

Detective Dave Brucz said, according to the Daily Mail: "Johnny began to attack the grandmother. Bjorge then said she had a surge of energy. She dragged her grandmother into her grandfather's bedroom. She was duct taped."

Officers have stated that Bjorge has confessed to the killings but Rider has not spoken since his arrest.

Authorities believe the duo decided to murder the pensioners because they "basically had enough" of them, it was said.

Brucz added: "She admitted after the murder she was texting family members because they were worried [about the grandparents], and she was pretending to be Wendy."

Police allege that the duo went on the run after attacking Bjorge's sister and her boyfriend with a baseball bat.

Bjorge's sister called police and the pair were later arrested, with authorities claiming they had planned to murder more family members, including Bjorge's mum.