A mother and her two daughters are believed to have died in a chemical poisoning at their home.
The woman, named in reports as Heena Solanki, 34, was found dead with her children Jasmine, nine, and Prisha, four, at their home in Ruislip, west London, on Friday.
Police said it was not a gas mains incident and the deaths are being treated as unexplained. It is understood that officers are not looking for anyone else in connection with the situation.
Paramedics were called to the property in Midcroft at 6.40pm, along with a hazardous area response team from the London Ambulance Service.
Mrs Solanki was reportedly a laboratory technician, and detectives are looking into whether she gassed herself and her children, the Sunday Telegraph said.
Police are said to have sealed off the house and ordered neighbours to remain inside with doors and windows closed.
Don Berry, 76, told the Sunday Mirror: "Police told us to shut the windows as there might have been a toxic substance.
"We didn't know what to think. We had no idea what was going on and no one would tell us."
Scotland Yard said detectives from the homicide and serious crime command were investigating, and that post-mortem examinations are expected in the coming days.