A one-year-old boy has died after being found injured in a north London flat.
Police called to Wilberforce Road in Finsbury Park on Saturday night also discovered a girl the same age who is in a critical condition.
The children, thought to be twins, were taken to an east London hospital after the discovery at 23:10 GMT. The boy died in the early hours of Sunday.
Police are trying to trace Bidhya Sagar Das, 33, who also lived in Wilberforce Road.
A neighbour told the BBC a woman came downstairs screaming for help, and another neighbour called the police.
One person living opposite the building, who gave her name as Gui Gui, said she had heard a woman shouting.
“I opened the window and I asked her ‘Can I help you, can I call the police for you?’
“She said, ‘My kids’.”
The neighbour said she later saw two young children being carried out of the building, with one being held very close to a member of the emergency services.
Mihai Menea, 29, who lives on the second floor of the house where the family lives, said the children were twins.
He said the father had been employed at the nearby Pembury Hotel, where Mr Menea also works.
Mr Menea said of the mood inside the property: “It’s really tense. There’s police coming and going all the time.”
Detectives said inquiries were under way to “establish the full circumstances”.
No arrests have been made, but the Metropolitan Police added: “Next of kin are aware.
“Formal identification awaits and a post-mortem examination will be scheduled in due course.”
A murder investigation is under way, police said.