Four people, including an 11-year-old girl, a three-year-old boy and a man and woman in their 30s, have been found dead in a flat in Hounslow, the Metropolitan Police said.
All four, who were found at the property in Staines Road, Bedfont, are thought to be related, the force said.
Police were called at 15:00 BST on Friday and found the bodies after forcing their way in.
The force described it as “a terrible incident”.
Ch Supt Sean Wilson, local policing commander in west London, said: “Our investigation is at a very early stage, and based on our initial inquiries, we are not currently seeking anybody else in connection with the incident.
“I know the shock and distress that this terrible incident will cause among the community in Hounslow and beyond.
“I can assure local people that specialist officers are working to establish exactly what happened and I will provide further information as soon as I can.
“I am also aware that this incident will be particularly upsetting for children, and I ask people to please be responsible about what they post on social media, including not naming or speculating as to the names of those who’ve died.”
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