Ohio authorities scrambled Saturday to establish who massacred eight members of a family in execution-style killings, as a businessman put up a US$25,000 reward for information.

Seven bodies, each with a bullet to the head, were found Friday at three houses in the village of Peebles – a rural community 80 miles (130 kilometers) east of Cincinnati – and an eighth later at a separate site, police said.

Other than one 16-year-old, the victims were all adults. Several of them were apparently asleep in bed when they were killed.

"Each one of the victims appears to have been executed, each one of the victims appears to have been shot in the head," Ohio Attorney General Mike DeWine said.

The killers appeared to have spared a baby just four days old, who was found lying next to her dead mother, the authorities said. Another baby aged six months and a three-year-old child also survived.

The victims were all members of the Rhoden family, authorities said, declining to provide any more details.

Officials gave no possible motive and said no arrests have been made.

A manhunt was underway for at least one suspected gunman, with authorities urging residents to lock their doors.

"There is a threat there and I believe that threat to be armed and dangerous," Pike County Sheriff Charles Reader said.

Investigators following up on an "overwhelming" number of tips had interviewed more than 30 people in connection with the killings by Friday evening.

"We do not know if we are talking about one individual or two or three or more," DeWine said.

Cincinnati-area businessman Jeff Ruby offered $25,000 for information leading to the arrest and conviction of anyone involved in case.


'Terrible thing'

It was not clear when the shootings took place.

The authorities were first alerted by an emergency call shortly before 8:00 am Friday reporting blood in a home with two possibly dead men, Reader said.

The Rhoden family issued a statement on Saturday asking "that everyone be respectful of their family of their loss at this time" and urging anyone with information to contact the authorities.

The shootings have shaken residents of Pike County, an economically distressed region of around 28,000 people in the foothills of the Appalachian Mountains that is home to an abandoned uranium plant.

"It's just a terrible thing," Sally McDaniel, a family friend, told ABC News. "You'd have to be a monster to go in and do something like that."

Ohio Governor and Republican presidential candidate John Kasich tweeted that the killings were "beyond comprehension."

Firearms kill around 30,000 people in the United States each year.

However, Republican lawmakers, some of whom are backed by the powerful National Rifle Association, have blocked President Barack Obama's attempts to pass gun control legislation.