Fumes from a gas boiler could have killed three people found at a house in east London, police have said.
A family friend raised the alarm
The bodies of a woman in her 30s and two boys aged three and 18 months were discovered at the property in Alderman Avenue, Barking, on Thursday afternoon.
Two women rescued from the house are in a critical condition in hospital.
Police said inquiries were focusing on work carried out on a boiler at the weekend. A man has been arrested on suspicion of manslaughter.
Det Ch Insp Ian Stevenson said: "One line of inquiry we are following is that over the weekend... some work was done to a gas boiler, the deaths may be connected to carbon monoxide poisoning."
Emergency services were alerted after a friend of the family discovered the victims inside the house.
Seven ambulance crews, an air ambulance and a fast response car with paramedic crew were called to the scene at about 1340 BST.
An ambulance spokeswoman said the two women rescued from the house were treated for breathing difficulties at the scene.
The arrested man is being held in custody at an east London police station.