Eight members of the Rhoden family were each found with a bullet to the head, in three separate homes on Friday.
Officials in Ohio say the assassination of eight family members was a 'sophisticated operation' that will require a lengthy investigation.
Police found seven bodies, each with a bullet to the head, in three separate homes on Friday.
An eighth body was found at a separate location.
The victims were asleep when they were shot.
Police named the victims as Hannah Gilley, 20; Christopher Rhoden Sr., 40; Christopher Rhoden Jr., 16; Clarence "Frankie" Rhoden, 20; Dana Rhoden, 37; Gary Rhoden, 38; Hanna Rhoden, 19; and Kenneth Rhoden, 44.
Ohio Attorney General Mike DeWine says the murders, which took place in the rural Ohio community village of Peebles was "well-planned out and thought-out."
"This is not your case where someone got mad at somebody else, they shot them, there's a witness, two witnesses. It's a very different type of case."
The shooters spared the life of a four-day-old baby who was found lying next to her dead mother.
An infant aged six-months and a three-year-old child also survived.
Officials say no arrests have been made and declined to indicate whether they had identified any suspects or to name any possible motives.
Pike County Sheriff Charles Reader says investigators are working "around the clock, 24 hours a day."
However, investigators had found marijuana grow operations, at three of the four locations, where the victims were killed.
Reader said the area had a drug problem, but added that he had never been involved with the assassinated family criminally, in his 20 years in law enforcement.
The 911 calls, released by the authorities, appeared to indicate at least some of the victims had been badly beaten.
Investigators had conducted between 50 and 60 interviews and received more than 100 tips, DeWine said, adding that they had also killed five search warrants, declining to say where.
The officials also declined to say whether the investigation extended beyond Ohio or to provide any more details.
Reader said he had advised surviving family members to protect themselves and be armed.
Cincinnati-area businessman, Jeff Ruby, has offered $25,000 for information leading to the arrest and conviction of anyone involved in case.