A one-year-old boy died of multiple stab wounds, a post-mortem examination has found.
The boy was found dead at a house in Beaford Road, Wythenshawe, Manchester, just after midnight on Sunday.
A man and woman left the house to go to hospital before police arrived.
A woman, aged 30, arrested on suspicion of murder remains in hospital, said police. A 31-year-old man also arrested on suspicion of murder has been released without charge.
Formal identification has taken place and the boy’s family are being supported by specially-trained officers, said police.
Det Insp Carl Jones of Greater Manchester Police (GMP) said it was a “truly distressing investigation”.
He said: “The family wish for their privacy to be respected at this time and I’d ask that people do not speculate around the boy’s identity to allow them to grieve and come to terms with this tragedy.
“We have one person under arrest, but need those who have not yet passed on information to come forward.”
The police force has referred itself to the Independent Police Complaints Commission as it had previously been called to the property.