Eight family members found shot to death at four homes in rural Ohio were specifically targeted for execution in a pre-planned, "sophisticated operation," authorities said on Sunday as their investigation of the unsolved killings entered a third day.

Revealing one possible clue in the case, officials said investigators had found three marijuana cultivation sites at one of the homes, but they declined to say whether the cannabis "grows" might be linked to the killings.

Pike County Sheriff Charles Reader and Ohio Attorney General Mike DeWine said it was clear the victims, ranging in age from 16 to 44, were deliberately singled out for attack, most of them while they slept, rather than killed at random or in a crime of passion.

In their latest briefing to reporters, however, officials refused to comment on any possible motive or say whether they had identified any suspects. While DeWine at times used plural pronouns, he insisted investigators remained unsure whether they were seeking one or more perpetrators.

"I don't know if it's a bad guy or bad guys - it could be one, two three, four," he said in a televised news conference. He said 18 pieces of evidence had been sent to a state crime lab for analysis and that 50 to 60 people had been interviewed in the case, with more expected to be questioned.

"This was a pre-planned execution of eight individuals. It was a sophisticated operation. And those who carried it out were trying to do everything they could do to hinder the investigation and their prosecution," DeWine said.

"We would anticipate that this could be a lengthy investigation," he added.

The victims, all previously identified as members of the Rhoden family, were found shot in the head in four separate homes on Friday in or near Piketon, a town of some 2,000 people about 95 miles east of Cincinnati in south-central Ohio.

The victims included a mother who was slain in bed with her 4-day-old infant in her arms. That baby and two other small children present during the killings - a 6-month-old and a 3-year-old - all survived, officials said.

The sheriff said he has urged other surviving members of the Rhoden family to arm themselves for protection, but "for other citizens of this county, I don't believe there is an issue."

Autopsies were completed on seven victims by Sunday afternoon. The sheriff also said he had sent additional investigators to scour the wooded areas surrounding all four crime scenes in search of any additional evidence.

A local restaurateur, Jeff Ruby, offered a $25,000 reward for any information leading to arrest and conviction of those responsible. – Reuters