A 13-year-old Pennsylvania boy charged with fatally shooting his grandparents in their home was upset because they called his mother a "whore" and were "speaking poorly about her," according to state police and court documents.

Zachary James Proper, of Oil City, was arraigned Tuesday morning as an adult before District Judge Michael Snyder in Franklin, Pa., on two counts of criminal homicide. He was remanded to Venango County Jail without bond, Pennsylvania State Police said.

Troopers found the bodies of Dorothy Fross, 67, and George Fross, 69, on Monday night in their home in Sandycreek Township in Venango County. They went there after the Frosses’ son – the boy’s father – called police to ask them to check on the couple’s welfare.

Proper's parents were concerned that their son had gotten drunk and high on Sunday night and had stolen a car belonging to his grandparents, police said in a criminal complaint, The Associated Press reported.

Proper lived about 10 miles away in Oil City with his parents but often spent time at his grandparents’ home, investigators said.

Police believe the Frosses were shot in their home late Sunday afternoon.

Watch US News crime videos on NBCNews.com

According to the police affidavit, the boy admitted to the killings, the Pittsburgh Post-Gazette reported. The affidavit outlined this sequence of events, according to the Post-Gazette and the AP:

Proper used bolt cutters to break into a gun safe at his grandparents’ home and took a handgun. He put on headphones, sneaked up behind his grandmother and fatally shot her. He then called his grandfather and told him that he would meet him at the front door to help him carry in groceries. When the grandfather came home, Zachary took a pizza from his hands and then shot him repeatedly until the gun jammed.

Asked to explain the shootings, the boy told police “that he was mad at his grandparents because they had called his mother a whore and were speaking poorly about her.” Police did not elaborate on the remarks.

The boy told police he then drove his grandparents' car to Oil City, dumped the gun in a storm drain and spent the night partying at a cousin's house, the Gazette and the AP reported, citing the affidavit.

More content from NBCNews.com:

Follow US news from NBCNews.com on Twitter and Facebook