A man has been arrested on suspicion of the murder of a boy and the attempted murder of a girl, both aged one.
Bidhya Sagar Das, 33, believed to be the children’s father, had been wanted in connection with an incident at a flat in Finsbury Park, north London.
The children, thought to be twins, were found at the flat with critical injuries on Saturday night.
The boy died in the early hours of Sunday and the girl remains in a critical condition in hospital.
Officers said they had been called to Wilberforce Road, in Finsbury Park, at 23:10 GMT on Saturday by a “distraught” member of the public following a “domestic incident”.
One child was taken to the Royal London Hospital by air ambulance and the other by ambulance.
Witnesses said they had heard a woman shouting for help on the street outside the Finsbury Park flat and two children were later seen being carried out of the property.
A woman living opposite the building, named as Gui Gui, said she had heard a woman shouting late on Saturday and opened the window to offer help.
Mr Das was arrested at 19:15 GMT on Sunday in Hackney, east London, and taken to a nearby police station where he remains in custody.
Det Ch Insp Dave Whellams, who is leading the investigation, said: “This is clearly a tragic incident, and our thoughts are with the family of the two young children.
“Despite the best efforts of medical professionals, a baby boy sadly died in the early hours of this morning.
“A baby girl currently remains in a critical condition and is receiving specialist medical care.
“Whilst we remain in the early stages of the investigation, a man has now been arrested.
“I would still like to hear from anyone who may have any information regarding this terrible incident.”