Police in Brixton said the man they have arrested following a fatal stabbing where a 28-year-old woman was killed trying to protect her younger sister, is known to them, but would not give any more details.
Speaking this afternoon, Detective Reynolds said young children were at the scene of both attacks, which involved "extreme violence", and officers are trying to establish whether they saw anything significant.
Police were called to a home in Redlands Way (marked 'A' below), Lambeth, in south London yesterday evening following reports that a women had been stabbed.
The 46-year-old victim was taken to hospital with multiple injuries, including stab wounds and remains in a critical condition.
Thirty minutes later, at approximately 7.10pm, officers were then called by London Ambulance Service to a home in Tilson Gardens (marked 'B' above), Lambeth, following reports that two women had been attacked.
The 28-year-old victim was pronounced dead at the scene and the 17-year-old girl was taken to hospital.
A 40-year-old man has been arrested on suspicion of murder following a triple stabbing incident in South London.
A 28-year-old woman has been killed, a 46-year-old woman is in a critical condition in hospital and a 17-year-old girl is in a serious but stable condition.
Investigating Officer DI Will Reynolds said police believe all three of the victims are known to the man in custody and the incident was not random.
Our investigation is at a very early stage, however we are confident that there are no outstanding suspects. We believe that all three victims were known to the man who is currently in our custody, there is no suggestion that the victims were selected at random.