The shootings that left eight people dead in rural Ohio were planned in advance and carried out with precision.

The investigation is going to be a lengthy process, Ohio Attorney Gen. Mike DeWine and Pike County Sheriff Charles Reader said at a press conference Sunday.

“This is a pre-planned execution of eight individuals," DeWine said. "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."

Investigators found marijuana grow operations at three of the four crime scenes. No arrests have been made as investigators work to solved the shooting deaths of eight members of the Rhoden family in Pike County.

“We don’t know if it was one or two” shooters, DeWine said.

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The victims were found Friday at four locations in Pike County, which is about 100 miles east of Cincinnati. Each victim was killed by an execution-style gunshot wound to the head. Three children, ages 4 days, 6 months, and 3 years, were found unharmed at the crime scenes.

“We have received over 100 tips. We have conducted over 50 to 60 interviews…Over 100 personnel were involved in this investigation. Five search warrants have been executed. Four crime scenes have been worked,” DeWine said.

Also 18 pieces of evidence were sent to the Bureau of Criminal Investigation’s state crime lab. Seven autopsies have been completed and the eighth will be completed Monday, DeWine said.

Reader said the family did not have prior criminal contact with his office.