Police say a 10-month-old baby was among three children who were found dead in a house in Bristol.
The children, which also included a boy, aged seven and a girl, aged three, were discovered following a "terrible and deeply disturbing tragedy".
Officers were responding to a welfare call in Blaise Walk, Sea Mills, at 12.40am on Sunday.
A 42-year-old woman has been arrested on suspicion of murder.
Police say a 10-month-old baby was among three children who were found dead in a house in BristolPAThe woman is being held in police custody at hospital in a non-life threatening condition.
Avon and Somerset Police said post-mortems will now take place to determine the children's cause of death.
Formal identifications of the victims are yet to be released, but their next of kin have been informed and are being supported by specialist family liaison officers.
Chief Inspector Hayward-Melen said police are treating it as an isolated incident and there is no ongoing threat to the wider community.
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Police are at the scene in Blaise Walk, in Sea Mills, Bristol
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Police confirmed the death in a press conference this morning
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She said: "The death of such young children is a great shock to the whole community, and this incident has had a profound impact on all of us in the police.
"We've been humbled by the community response to this tragedy. At a time of great sadness and disbelief, we've seen spontaneous acts of care and support.
"Within hours of the incident occurring, a local church was opened up for people to gather and mourn.
"This is what being in a community is all about and we'd like to thank all those who continue to offer this important and selfless service."
Avon and Somerset Police said post-mortems will now take place to determine the children's cause of death
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She added that detectives are in the early stages of a "highly sensitive, complex and thorough investigation".
The Independent Office for Police Conduct (IOPC) has been notified of the incident due to "prior police contact earlier this month".
An IOPC spokesperson said on Monday: "We have requested a paper referral with further information about the prior contact and once received, we will assess it to determine whether further action is required from us."