A major search for a "suspect" is under way after a one-year-old boy died and a girl of the same age was left in a critical condition.
Officers were called to reports of two children injured in a flat in Wilberforce Road, near Finsbury Park, in north London at 11.10pm on Saturday, Scotland Yard said.
Both toddlers were found with critical injuries and were taken to an east London hospital where the boy died in the early hours of Sunday morning.
The girl remains in a critical condition.
The Metropolitan Police said: "Next of kin are aware. Formal identification awaits and a post-mortem examination will be scheduled in due course."
They added that inquiries were under way to "establish the full circumstances of the incident" and said that no arrests had been made.
The National Police Air Service helicopter tweeted that it was assisting "officers searching Finsbury Park for a suspect".