"We got a call about 9 a.m. that a family member was checking on her mom and dad and found them both passed away in the garage," Huron County Sheriff's Chief Deputy Ted Patrick said.
Patrick said deputies secured the scene and notified the state Bureau of Criminal Investigation.
Patrick said the deceased were Dr. Donald King and his wife, Lucille, who were both reported to be in their 80s.
"We investigate all deaths now as a homicide," Patrick said. "That's our standard policy.
"Both were found in the garage and the car had been turned on and the garage doors had been barricaded with a lawnmower and car," Patrick said.
"They both passed away from asphyxiation," he added.
"The fire department was on the scene with a carbon monoxide meter and noted carbon monoxide levels 10 times more than the human body can tolerate inside the garage and home," Patrick said.
Patrick said the Kings had last been seen two days ago.
A doctor of osteopathic medicine, King had a general practice in Norwalk. He received his medical degree from Des Moines University's College of Osteopathic Medicine in Iowa. King had 56 years of experience in the medical field.
Huron County Coroner Dr. Jeffrey Harwood could not be reached for comment Monday. Harwood rules on the official cause of death.