Two sisters and a man found murdered in a flat had been tied up before being shot, police said.
A family friend found the bodies
Officers named the victims as Lorna and Connie Morrison, 34 and 27, and their mother's partner Noel Patterson, 62.
Lorna's nine-month-old son, Christiano, was also in the flat in Stonebridge, north-west London, but was unharmed.
Police said they were keeping an open mind about the motive but added they want to speak to three youths seen entering the flat on Thursday.
Det Supt Gary Richardson said the sisters' mother, Evelyn Daley, was out working as a carer on Thursday night when the killings are believed to have taken place.
A family friend arrived at the flat to find the front door open and the three victims downstairs in the main living quarters.
Mr Richardson said: "It would appear that the victims had been incapacitated by the use of restraints."
But he refused to give more details on exactly how this had been done.
He added: "We don't know what the motive was, that is one of our main lines of inquiry, to find out why these people were targeted if indeed they were targeted."
He said close inspection of the wounds had not taken place but it was likely that the three had been shot in the head.
He said officers were not ruling out the possibility that the killings could have been the result of a burglary or robbery.
Mr Richardson said the three youths they want to question were not being treated as suspects at this stage.
Two of them are described as black, being around 6ft tall and one considerably shorter. They were wearing hooded tops.
The bodies of the three victims remained at the scene this afternoon, in the small two-bedroom flat in which five people had lived.
Mr Richardson said it was believed a semi-automatic weapon, or weapons, had been used because of the presence of cartridges, but detectives had not yet established how many guns were involved.