A Bolton couple have been arrested on suspicion of murdering their six-week-old son.
The youngster died at Alder Hey Hospital in Liverpool on Saturday morning after he was transferred from the Royal Bolton Hospital.
The boy's parents, a 22-year-old woman and a 24-year-old man, are being questioned by detectives as further tests take place to establish the exact cause of death.
Police are not looking for anyone else in relation to the incident.
Detective Chief Inspector Dave Warren, from Greater Manchester Police's Major Incident Team, said: "Tragically a very young baby has died and our thoughts are with his family and friends.
"We are in the very early stages of this investigation but I want to reassure all those close to the baby and the local community that we are doing everything we can to establish the full circumstances leading up to and surrounding his death."
The couple are also being quizzed on suspicion of causing or allowing the death of a child. The baby had been admitted to hospital after paramedics were called to the family home in Heaton, Bolton, last Thursday.