Two men have died after collapsing in a sauna on Saturday, Scotland Yard said.
A 39-year-old man was pronounced dead at the scene in the Pleasuredrome sauna near Waterloo station in central London, while a 46-year-old man was taken to hospital but died on Monday.
A third man, aged 34, was taken to hospital for treatment and discharged later.
Police had been called to Pleasuredrome by the London Ambulance Service at around 8.15am on Saturday, following reports that three men had collapsed.
Police said a post-mortem examination of the 39-year-old failed to establish a cause of death, but the force was awaiting results of further toxicology tests.
A post-mortem examination of the 46-year-old will take place on Thursday.
Next of kin have been informed although formal identification has not taken place. Police are treating both deaths as unexplained.
Officers in Lambeth are appealing for a man seen entering the venue with the 39-year-old man to come forward as soon as possible.
He is described as a white man, aged between 25 and 35 with brown hair and a goatee beard, approximately 6ft with a slim build.
Police said they were keen to identify and trace this man they have concerns for his welfare.