Police are investigating how and why a three-year-old boy, a woman in her thirties and a man in his forties died at a home in Brentford, west London.
The man was found suffering stab wounds when officers forced their way into the address just before 1am. They also discovered the bodies of the woman and the boy.
The Metropolitan Police said they were not looking for anyone else in connection with the incident.
Officers said they broke into the home, in Clayponds Lane, “following concerns for the welfare of the occupants”.
London Ambulance Service workers attended but the man was pronounced dead.
Police say they believe all three “were known to each other” and that relatives were being informed of the deaths.
A Met Police statement said: "Enquiries are under way to inform all next of kin.
"Post-mortem examinations will be scheduled for all three people in due course.
“Detectives from the specialist crime command are investigating.”
