A man has been arrested on suspicion of murdering a young child in Darlington.
Police were called to Frosterley Drive at around 9am on Friday.
The one-year-old was found at the address and rushed to the Royal Victoria Infirmary in Newcastle but died on Saturday afternoon, Durham Police said.
A woman in her 40s has been arrested on suspicion of causing or allowing serious injury to a child.
The man, who is in his 30s, has also been arrested on suspicion of causing or allowing serious injury to a child.
Both have been released on conditional bail while further inquiries are carried out, the force said.