A laboratory technician is feared to have killed her two young daughters with a homemade toxic gas, before taking her own life.
Heena Solanki, 34, is believed by police to have used her knowledge of chemistry to poison her daughters Prisha, four, and Jasmine, nine, at their home.
All three were found dead at the house in Ruislip, west London, which Mrs Solanki shared with her husband Kalpesh, 42, and his parents.
Officers said yesterday that they were not seeking anyone else in the investigation into the three deaths.
Detectives are looking into whether Mrs Solanki concocted a poison gas.
"They were a lovely family and the children were brought up very well," said a woman living nearby, who asked not to be named. "They always made an effort to greet you. It's all very sad."
Another neighbour said: "The kids used to run around and play outside, always very happy. My wife is very upset.
"It's heartbreaking. They were lovely children. The mother worked in a lab."
The girls' father is understood to have worked away at times and was not at home when the alarm was raised at 6.40pm on Friday.
Moments after paramedics and police had arrived at the house, they ordered neighbours to close all their doors and windows, saying that there had been a "chemical incident" there.
It was not clear last night whether Mrs Solanki's parents-in-law, Chaganlal, 71, and Kalavati, 61, were at home at the time of the deaths.
Floral tributes were placed outside the semi-detached property after the three bodies were removed in a private ambulance yesterday following forensic examinations.
A spokesman for Scotland Yard said that detectives from the homicide and serious crime command were investigating.
He added that post-mortem examinations were likely within days.