Police have named a man they are hunting after the death of a one-year-old boy and serious injuries to his twin sister.
Bidhya Sagar Das, 33, is from the address in Wilberforce Road, north London, where the injured children were found on Saturday night.
Police said: "Detectives are urgently trying to trace a man who was at the flat shortly before the injured children were found, but left before emergency services were called.
"The man has not been seen since this time."
Police found the twins seriously injured after being called to the flat near Finsbury Park at 11.10pm on Saturday.
An ambulance took them to hospital, but the boy died just before 1am on Sunday.
The girl remains in a critical condition.
Detectives from the Homicide and Major Crime Command have launched a murder investigation and say the incident is believed to be domestic.
No arrests have been made so far.
Police appealed for anyone who knows where he is to contact them by phoning 999 immediately.
Mr Sagar Das is understood to have worked at a nearby hotel, but a colleague told Sky News that he had been absent for the last two weeks.
At the scene, Sky News reporter Charlotte Lomas said: "I've been speaking to neighbours here who have told me that they heard a woman screaming in the early hours, shouting out for help.
"They've also been telling me that police have been knocking on their doors asking to do a quick search of their property as part of their ongoing enquiries."
A National Police Air Service helicopter tweeted on Sunday morning that it was helping "officers searching Finsbury Park for a suspect".
A woman living opposite said she had heard a woman shouting last night and opened the window to offer help.
"I was watching TV," she said. "I heard someone was shouting. She kept on shouting...
"I opened the window and I asked her 'Can I help you, can I call the police for you?'
"She said, 'My kids'."
Mihai Manea lives on the second floor of the building where the children were found and said the mood at the property was "tense".
He added: "There's police coming and going all the time."
A post-mortem examination on the boy is due to take place soon and police said his family had been informed of his death.