A one-year-old girl is fighting for her life as police question a man arrested on suspicion of her attempted murder and the murder of a boy believed to be her twin.
Bidhya Sagar Das, 33, was wanted by police in connection with a "domestic incident" at a flat in Wilberforce Road, near Finsbury Park, north London, on Saturday night.
Both toddlers were discovered with critical injuries at the address and were taken to an east London hospital, where the boy died in the early hours of Sunday morning.
Das was arrested in the Hackney area at 7.15pm on Sunday following a major search by police. He was taken to an east London police station where he remains in custody.
The girl, who has been operated on, remains in hospital in a critical condition, police said.
Witnesses heard a woman shouting for help on the street outside at around 11.10pm on Saturday and later saw two children being carried out of the property.
Mihai Manea, 29, who lives on the second floor of the white three-storey building, said the children were twins and lived on the top floor with their Romanian mother and Indian father.
A woman living opposite the building, who gave her name as Gui Gui, said she opened the window to offer help after hearing a woman shouting.
"I was watching TV," she said. "I heard someone was shouting.
"She kept on shouting. I do not know what she was shouting. I opened the window and I asked her 'Can I help you, can I call the police for you?'
"She said, 'My kids'."
She later saw two young children being carried out of the building, with one held very close to a member of the emergency services.
The incident is believed to be domestic and enquiries continue, police said.
Next of kin have been informed of the boy's death but formal identification and a post-mortem examination are yet to take place.
Detective Chief Inspector Dave Whellams, of the Metropolitan Police, 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."