Two men have died after a possible carbon monoxide leak from a faulty boiler.
The pair, thought to be aged 38 and 42, were found at an address in Edgware, north London, on Sunday afternoon.
Five other people were taken to hospital as a precaution. The occupants of the rented multi-occupancy semi-detached property are thought to have moved in just weeks ago.
Scotland Yard said the deaths are being treated as “unexplained” but police believe there was a possible carbon monoxide leak at the property on Bacon Lane.
Officers were called at around 1.30pm to reports of two unresponsive males, finding both men dead when they arrived along with the London Ambulance Service and London Fire Brigade.
The Metropolitan Police said no arrests have been made.
“At this early stage, police believe that there was a possible carbon monoxide leak at the address,” the force said in a statement.
“Police in Harrow continue to deal alongside the Health and Safety Executive who have been informed.”
London Fire Brigade said units rushed to the scene at 1.42pm on Sunday.
A spokeswoman said: “We have had two males pronounced life extinct by London Ambulance Service.
“Two adult males, one adult female, one male child and one male infant have also been treated on site for carbon monoxide poisoning and taken to hospital by London Ambulance Service.”
Neighbours spoke of their shock at the deaths. One neighbour, who thought they were from Eastern Europe, said: “It’s really shocking, very sad, the loss of life. It doesn’t matter how it happens, but it’s really sad.”
The man, who works in the medical profession, said: “It’s a rented property. I know the owner, I’ve met him.
“He was renovating just after Christmas. I don’t know what they did inside, but I think they did carpets, painting.”
Another neighbour said the residents had only been living in the house for three to four weeks.
“They are fairly new. As far as I know they’re very quiet,” he said.
“They have only been here a short period of time, they have been very quiet.
“I have never spoken to them, they kept themselves to themselves.”
The windows of the house were left open on Sunday evening, while a gas engineer arrived to reconnect the gas to neighbouring houses.