COLUMBUS, Ohio — The slayings of eight family members in rural southern Ohio was a planned execution, authorities said Sunday, telling residents they are safe but can arm themselves if they are fearful.

The Friday killings at four homes near Piketon was ‘‘a sophisticated operation,’’ Attorney General Mike DeWine said at a news conference in the small community that has been on edge since seven adults and one teenage boy were found shot in the head.

Authorities remained tight-lipped Sunday about details of the investigation, and whether there are any suspects or motives for the crime. They said they found marijuana operations at three of the crime scenes, but didn’t say if the deaths were linked to drugs.

Investigators finished collecting evidence at the four houses but said the crime scenes will remain under law enforcement custody, at least until autopsies are completed.

The investigators said they had interviewed more than 30 people in connection with the attack.

Flags flew at half-staff at the government center in Piketon, where residents were fearful. ‘‘I know a lot of people are just scared,’’ said Maggie Owens, a cook at the town’s Riverside Restaurant. ‘‘You don’t hear about stuff like that around here.’’

Governor John Kasich said Sunday he is confident that the perpetrators would be found.

ASSOCIATED PRESS