Three men have been found dead at addresses in Lancashire within about an hour.
The men, aged 52, 36, and 32, were found in properties in Burnley, Colne and Nelson between 2315 GMT on Friday and 0020 GMT on Saturday.
Police have said they are treating the deaths as unexplained but are concerned they may be linked to a batch of heroin in the area.
Anyone with information is asked to contact police or Crimestoppers.
Det Ch Insp Stuart Dixon, of Lancashire Police, said: "It is too early to say whether the cause of the deaths are connected, but we are concerned that there could be a particularly strong batch of heroin in the area and I would urge anyone who uses heroin and who may be concerned to contact their local healthcare provider.
"We have circulated a warning to our partners in the healthcare agencies and those that offer support to drug users."