A man has been arrested on suspicion of the murder of a one-year-old boy and the attempted murder of a girl, believed to be his twin.
Metropolitan Police had been searching for Bidhya Sagar Das, 33, in connection with the investigation.
He was arrested in the Hackney area of London at 7.15pm on Sunday night.
The suspect was taken to an east London police station where he remains in custody.
Officers found the children seriously injured after being called to a flat on Wilberforce Road 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.
Detective Chief Inspector Dave Whellams, of the Homicide and Major Crime Command, said: "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."
Anyone with information that may assist the investigation is asked to contact the incident room on 020 8345 3775 or Crimestoppers on 0800 555 111.