Investigators who came to the house for a welfare check Tuesday afternoon found three children huddled inside an attached garage. The children were unharmed, police said.
But inside the house, authorities found a grisly scene. Sharon Reed, 59, and James Reed, 78, were dead.
"Both victims had been shot and decapitated," police said
Officers had spotted Quinton Laster, 20, outside. They detained him there as they ventured into the house. Now, police say Laster, the Reeds' grandson, is being held on two counts of first-degree murder.
Laster was silent as a judge addressed him during a court appearance Thursday. Once a district attorney files charges in the case, the judge said, a hearing will be scheduled and Laster will be assigned a court-appointed attorney.
Investigators haven't revealed details about a possible motive in the killing, or outlined any evidence they've uncovered in the case.
Family members told CNN affiliate KFOR
that they were devastated, and they didn't know what triggered the killing.
"It's just a shock," said Brenda Jackson, Sharon Reed's sister.
"He didn't deserve it, and she didn't deserve it," Jackson said.
Laster had been living with his grandparents for about two years after being kicked out of his mother's house, Jackson said.
Images from KFOR
showed police cars, an ambulance and police tape strung all around the stand-alone, single-family home.
The crime scene was nearly a block wide, the CNN affiliate reported.
The children found inside the garage were later reunited with their families, police said.
"Luckily, they weren't injured in any way, shape or form," Oklahoma City Police Sgt. Ashley Peters said.