"Lying there with their eyes wide open! Cold as ice! Still in their dinner things!"
-- Maid at the Riddle House after finding the bodies in the drawing room.
Grandparents of Tom Riddle Jr. (Voldemort) and parents of Tom Riddle Sr.
Mr. and Mrs. Riddle were wealthy landowners whose mansion overlooked the small village of Little Hangleton. They, along with their son Tom Riddle Sr., were found murdered in their house in the summer of 1943.
The elder Riddles were murdered by their grandson, Tom Riddle Jr., at the same time that he murdered his maternal grandfather and created a Horcrux using the Peverell ring.
Other canon notes and references
It's interesting to view the effects of a Killing Curse from the point of view of Muggle investigators. Rowling describes it with her usual pithy humor:
A team of doctors had examined the bodies and had concluded that none of the Riddles had been poisoned, stabbed, shot, strangled, suffocated, or (as far as they could tell) harmed at all. In fact (the report continued, in a tone of unmistakable bewilderment), the Riddles all appeared to be in perfect health -- apart from the fact that they were all dead. The doctors did note (as though determined to find something wrong with the bodies) that each of the Riddles had a look of terror upon his or her face -- but as the frustrated police said, whoever heard of three people being frightened to death? (GF1)
The Riddle family is likely part of the "landed gentry," a family which lives in a large home and owns much of the surrounding land. This fits with what Tom Jr. is says when his companion asks about the Gaunt shack:
"My God, what an eyesore!" rang out a girl's voice, as clearly audible through the open window as if she had stood in the room beside them. "Couldn't your father have that hovel cleared away, Tom?"
"It's not ours," said a young man's voice. "Everything on the other side of the valley belongs to us, but that cottage belongs to an old tramp called Gaunt, and his children. The son's quite mad, you should hear some of the stories they tell in the village —" (HBP10)