Four suspects have been arrested after two women were found dead at properties less than a mile apart.
Both women were discovered dead in homes within hours of each other in Yorkshire - first on Monday evening, then the second on the following morning.
Two men and two women have now been taken into custody on suspicion of administering a noxious substance and are being questioned by detectives.
Police described both deaths as "unexplained" after one woman was found in Ajax Way in South Bank, Cleveland, while the second was discovered around the corner in South Terrace.
Officers were called to the first home at about 5pm over fears for someone inside - and when they arrived found a woman dead.
Then at about 10.30am the next morning, police were called to "a sudden death" at a nearby property where a second body was uncovered.
Two 47-year-old men, plus two women aged 39 and 43 have been arrested, with all four still in custody.
Detectives have not yet officially linked the deaths and only one suspect has been arrested in connection with both incidents.
The 43-year-old woman has been arrested in connection with both deaths, while the three other suspects have been arrested over the body found on Tuesday, police said.
Formal identification of the two women has not yet taken place, but the families have been told of their deaths, Cleveland Police said.
A spokesman for the force said: "Police were called to a property in Ajax Way in South Bank on the evening of Monday, 18 September, following concerns for an occupant of the address.
"Sadly, the body of a woman was found inside the building.
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"Around 10:30am on Tuesday 19th September officers were called to a report of a sudden death at a property on South Court in South Bank where a second woman had passed away.
"Enquiries into both deaths are continuing and, at this time, police are treating the deaths as unexplained.
"Whilst formal identification has not yet taken place, the families of the two women have been informed.
"Two men aged 47 and two women aged 39 and 43 have been arrested at this time. All remain in police custody.
"A number of officers are in and around the area conducting enquiries and providing a reassuring presence."