The deaths of six people in suspected drugs-related circumstances all happened in their own homes, police have said.
Essex Police said three men and three women died in three days in the south of the county, between Sunday and Tuesday.
Chief Superintendent Kevin Baldwin said it is believed class A drugs are linked to the deaths and forensic work is continuing to identify the substance or substances.
Police say it is too early to say whether the substances were injected, smoked or taken as pills.
He cautioned against describing the drugs as a "bad batch" while the nature of the substance is not known.
"Descriptions of 'contaminated' or 'dodgy' may make drug users think slightly differently and it could be that the purity of the drug has caused this," he said.
"I don't know at all, but until we know for sure what the substance is, we don't want to give out a message that there's a contaminated sample out there or a dodgy sample as it may not be the case.
"It may well be that the samples that they ordinarily take are the ones that caused this as well."
A woman in her thirties was found dead in Southend on Sunday morning. On Monday, a man in his twenties was found dead in Leigh-on-Sea and a woman in her thirties died in Westcliff.
On Tuesday, a man in his forties died in Canvey Island, a woman in her forties died in Canvey Island and a man in his twenties was found dead in Benfleet.
Police have urged anyone considering taking class A drugs not to do so. Public Health England urged drug users to be "extra careful about what they are taking".
Anyone with information about the deaths or the sale of class A drugs is asked to call Essex Police on 101 or Crimestoppers anonymously on 0800 555 111.