Each boy had a gunshot wound to the head, Jackson County coroner Dr. Gregory P Hawker, told media outlets. It's unclear whether the first shooting was an accident.
Jackson City Police identified the boys as Austin Wiseman and his younger sibling, Blake Campbell. Police said they lived at their grandparents' home, where the shootings occurred Wednesday in an upstairs bedroom. Investigators don't believe anyone else was involved in their deaths.
Authorities have said that the grandparents were away at the time of the deaths, and that a .44-caliber handgun was recovered.
The boys' bodies were taken to Franklin County, some 65 miles north in Columbus, for autopsies, and the Ohio Bureau of Criminal Investigation was examining evidence collected from the home as authorities tried to determine what happened.
A man at the home who identified himself as an uncle said he no comment Thursday afternoon. The two-story house is set back slightly from the road in an older neighborhood with a mix of residential houses and businesses.
Hawker told The Associated Press on Thursday that he couldn't comment on what might have led to the shootings, but authorities say no one else was at home at the time.
Shirley Causey, who said her grandson was a cousin and frequent playmate of the older boy, said Austin was "very protective" of his younger half brother. She described Austin as so polite that he wouldn't get a drink or snack without asking her permission when visiting.
She said her grandson, Aaron Causey, was close to Austin and that the two enjoyed swimming, skating and riding bikes together.
"He (Aaron) is doing better, but he was just so torn up yesterday," she said.
Property records show the home is owned by James and Deborah Harris, and neighbors said they were the boys' maternal grandparents.
The grandparents were very loving to the boys, Shirley Causey said.
"They are just very sweet people," she said. "They just loved those boys so much."
There was no suspicion that anyone else was involved, but any shooting must be treated as a potential homicide until investigation rules that out, Hawker said. He said full investigation results might not be known for six weeks or more.
The boys were found Wednesday morning.
Hawker said the grandparents had left the home to take care of some personal business but had talked to the boys by phone at about 10 a.m. It's believed the shootings happened at about 11 a.m. Authorities didn't release the grandparents' names immediately.
Jackson is a city of about 7,000 people in mostly rural Appalachian Ohio.
People at the Alter Ego beauty salon near the Harris house said people were somber about the news.
"It's just a sad thing," said owner Pat Maynard.
Police Chief Carl Eisnaugle said in a statement that the community was deeply saddened, "and our sympathies go out to all those who have been affected."