A 31-year-old man has been arrested on suspicion of murdering a young boy.
Police were called to a report of a domestic incident in Wythenshawe, Manchester, at about 12.05am on Sunday and discovered the body of the youngster when they entered the property.
Officers also learned that a man and a woman had left the address in Beaford Road to go to hospital.
The woman is being treated for serious injuries in hospital while the man is being questioned in custody.
The victim has not yet been formally identified and a post-mortem examination will take place later.
Superintendent Denise Worth, of Greater Manchester Police, said: "I know the community will be horrified and saddened to hear this tragic news this morning, and our thoughts at this time are with the family, friends and all others within the local community who may be affected by this incident.
"I want to offer my assurances that we have launched a full murder investigation and have a team of detectives working to establish exactly what led to this young boy's death.
"Although we have made an arrest, the investigation is at the early stages and I would urge people not to speculate on this deeply-distressing incident.
"There will be a scene at the property whilst officers investigate and you will see more police in the area in the coming days.
"If you have any information to pass on, or you have concerns, please get in touch with one of our officers."
Anyone with information should contact police on 0161 856 2239, 101 quoting incident number 14 of 27 August 2017, or the independent charity Crimestoppers, anonymously, on 0800 555 111.
A cordon is in place around the modern semi-detached house in a quiet residential area.
A police van is also outside the address, which appeared to be a rental property.
Also within the cordon was a Volkswagen Polo car, parked in the drive.
Neighbours said they were woken by the sounds of sirens from police cars and ambulances shortly after midnight on Saturday.
One said: "I have seen a little boy play outside from time to time but I have never spoken to anyone who lives there. It's a shock because this is a really quiet street."
Another said they thought a man, woman and child had lived at the address for about three months.