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 having undergone surgery.
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 want to trace Bidhya Sagar Das, 33, who also lived in Wilberforce Road.
Det Ch Insp Dave Whellams, who is leading the investigation, said Mr Sagar Das left the address before emergency services were called.
“I would like to urge the help of the general public in trying to trace the following male… Any information as to this man’s whereabouts I would urge the public to phone 999 as a matter of urgency,” he said.
He added that police had been called to the house by a “distraught” member of the public, where they discovered the two badly injured children less than 18 months old.
“They were treated at the scene and taken to a couple of hospitals in east London, subsequently one of the children died, despite the valiant efforts of the medical staff. The other child remains in a very critical condition having undergone surgery,” he said.
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 a murder investigation was under way and they were trying “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.”