A man has been arrested on suspicion of murdering a one-year-old child in Darlington.
Police were called to Frosterley Drive at about 9am on Friday.
The child was rushed to Royal Victoria Infirmary in Newcastle but died on Saturday afternoon, Durham Constabulary said.
The suspect, in his 30s, has also been arrested on suspicion of causing or allowing serious injury to a child.
A woman in her 40s has 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 added.