ANOKA (AP) — Law enforcement authorities in Anoka County are investigating six separate heroin overdoses, two of them fatal, that happened in the span of 12 hours Saturday.

The Anoka County Sheriff's Office says investigators are speculating that the incidents could be linked to the same source of the drug.

Five men and a woman overdosed. Their identities have not been released. Two are from Fridley, while the others are from Anoka, Andover, Isanti and Ham Lake.

The sheriff's office says the number of cases and the time frame in which they happened is "alarming." It says the investigation is "extremely difficult" because heroin users do not cooperate with authorities.