Police have started a murder inquiry after a 17-month-old baby boy, who was taken to hospital with serious head injuries, died.
The baby was taken by ambulance to Manchester Royal Infirmary on Thursday evening, and later transferred to the Royal Manchester Children's Hospital.
Two men and two women were arrested on suspicion of assaulting the baby, who died on Friday evening.
None of the four, who remain in custody, are the baby's parents.
The men - aged 28 and 38 - and women - aged 27 and 28 - were arrested in Salford on Thursday.
The boy was taken to hospital at 1730 GMT and the arrests were made about four hours later.
Their exact relationship to the boy is not yet known and inquiries are continuing, Greater Manchester Police said.