A fifth person has died in a series of drugs-related deaths within a five-mile (8km) radius in the space of 36 hours.
Essex Police said the woman in her 40s was found dead on Canvey Island, Essex, on Tuesday.
It followed the deaths of three men and a woman in Leigh-on-Sea, Westcliff-on-Sea, Canvey Island and Benfleet.
Essex Police said the deaths were an “obvious concern” and were possibly linked to the taking of Class A drugs. It warned against taking them.
Det Ch Insp Stephen Jennings, from the Kent and Essex Serious Crime Directorate, said officers were “working hard” to establish if the deaths were linked.
He said: “Part of that work is to identify exactly what the substance or substances involved are and more tests are needed to do that.
“At this stage I believe the deaths may be linked to the taking of Class A drugs and I would urge anyone considering taking them not to.”
He also urged anyone with information about the sale of Class A drugs in the county to contact police.
On Monday, a man in his 20s was found dead at an address in Leigh-on-Sea and a woman in her 30s died in Westcliff-on-Sea.
Earlier on Tuesday, a man in his 40s died at an address on Canvey Island and a man in his 20s was found dead in Benfleet.