A 31-year-old man has been arrested on suspicion of murder after the body of a toddler was found by police.
The youngster was found dead at a house in Beaford Road, Wythenshawe, Manchester, at about 00:05 BST.
A man and a woman, who suffered serious injuries, had left for hospital before officers arrived following a report of a domestic disturbance.
A post-mortem examination into the boy's death is due to take place. The arrested man is in police custody.
Police are waiting to speak to the woman who is in a serious condition in hospital.
It has emerged officers had been to the address on Friday after a similar incident was reported.
Supt John-Paul Ruffle, of Greater Manchester Police (GMP), said the investigation into that domestic disturbance was continuing.
Police have cordoned off the property while investigations take place and more officers are expected to be in the area over the coming days.
Neighbours said they were woken by the sounds of sirens from police cars and ambulances.
Paul Dean said: "It's a very quiet road, this is the first time in the 10 years I've been here that something like this has happened.
"To find out it's a child involved, a very young child, it's horrible, a horrible feeling."
Another said: "I have seen a little boy play outside from time to time, but I have never spoken to anyone who lives there."
Another added they thought a man, woman and child had lived at the address for about three months.
Supt Ruffle said: "This is an absolutely tragic case and I know the community will be horrified and saddened to hear the tragic news this morning.
"I want to offer my reassurances that we have launched a full murder investigation and have a team of detectives working on this case to establish exactly what has led to this young boy's death."
Greater Manchester Police will be referring itself to the Independent Police Complaints Commission as it had previously been called to the property.