A woman is under police watch in hospital following the death of a nine-year-old girl and reported assaults on two other children.
The girl was pronounced dead on Tuesday at an address in the village of Islip, Oxfordshire.
Another girl and a boy were treated in hospital and have since been discharged, Thames Valley Police said.
A 49-year-old woman was taken to hospital in a life-threatening condition, the force said. No one else is being sought in connection with the incident.
Police received reports of assaults on three children at the property in Middle Street at about 11:15 BST.
The girl was pronounced dead at the scene, although the cause of death remains unascertained, the force said.
Det Insp Amy Fox said: “This is an unimaginable tragedy, and we are doing everything we can to support those affected.
“A 49-year-old woman from Oxfordshire was taken to hospital in a life-threatening condition and is currently under police bed watch.
“The scene has now concluded at the property, but officers will continue to be present in the area over the weekend, and we encourage anyone with concerns or information to speak with them.
“This remains a highly sensitive and complex investigation. I want to sincerely thank the community for their understanding and patience as we work to uncover the full circumstances.”
Previously, police appealed for anyone with information or camera footage from Middle Street to come forward.