At least eight members of a family, seven adults and one teenager, died in “execution-style” killings in four separate homes in rural Ohio.
They were all shot in the head and police are still searching for the attacker, or attackers.
More than a dozen officials from multiple agencies were sent to crime scenes in Piketon, south of Columbus.
A pastor at the scene said the violence may have been the result of a “domestic situation”.
According to Pike County Sheriff Charles Reader, all of the victims are members of a family called Rhoden.
They were all found in homes along Union Hill Road in Pike County. The Pike County Sheriff said there are four active crime scenes spanning about 30 miles.
Some of the victims appeared to have been shot as they slept, police said, including a mother in bed with her four-day-old baby close by.
Three children, including the infant, survived the shootings. The boy who was killed was 16 years old.
“There is a strong possibility that any individual involved in this is armed and incredibly dangerous,” Sheriff Charles Reader said.
Police have not determined a motive or officially identified the dead. They say none of the deaths appeared to be self-inflicted and they believe those responsible are still at large.
Ohio Governor and Republican presidential hopeful John Kasich has promised that those responsible will be found.
“We’ll find them, we’ll catch them and they’ll be brought to justice,” John Kasich said, while campaigning in Connecticut.
Sheriff Charles Reader said he would “suspect the family was being targeted”. He said he had met about 100 relatives of the victims at a church and they were being offered protection.
Ohio Attorney General Mike DeWine said it was “hard to believe” someone could kill a mother with her baby sleeping nearby.
Authorities are urging Pike residents to come forward with any information they might have.