At least eight people have been killed in "execution-style killings" in four places near each other in rural Ohio.
It is believed the victims - seven adults and one teenager - are from the same family, the state's attorney general said in a statement.
They were all shot to death in the head and any suspects are still at large, police said.
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".
All of the victims are members of a family called Rhoden, said Pike County Sheriff Charles Reader.
Three young children 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," Mr Reader said.
Police have not determined a motive or identified the dead, and have not determined whether the killer is among the deceased.
All of the victims were 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 (48km).
Sheriff Charles Reader said he would "suspect the family was being targeted".
Ohio Attorney General Mike Mike DeWine said it is possible some of the victims were shot overnight because they were found in their beds.
"One mom was apparently killed in her bed with [the four-day-old child] right there," said Mr DeWine. "It's hard to believe."
Authorities do not believe any of the deaths were suicides and are urging residents of the county to come forward with any information they might have.
Local schools Peebles Elementary and Peebles High School were earlier on "lockout" - no-one went in or out - due to the ongoing situation in Piketon, a spokesperson for Adams County Ohio Valley Schools said.
The FBI in Cincinnati tweeted that they are "closely monitoring the situation".
Ohio Governor John Kasich and Republican presidential candidate tweeted that the situation is "tragic beyond comprehension".