Three young children found dead at a house in Bristol were a 10-month-old baby boy, a three-year-old girl and a boy, aged seven.
Police officers discovered the children after responding to a welfare call in Blaise Walk, Sea Mills, at 00:40 GMT on Sunday.
Ch Insp Vicks Hayward-Melen said it was a “terrible and deeply disturbing tragedy”.
A 42-year-old woman has been arrested on suspicion of murder.
Forensic post-mortem examinations are set to take place later this week to establish the causes of death, Avon and Somerset Police said.
The woman is currently held in police custody at hospital in a non-life threatening condition.
Formal identification of the victims is yet to take place, but their next of kin have been informed and are being supported by specialist family liaison officers, police said.
In a statement read out by Ch Insp Hayward-Melen earlier, the force appealed for information from the public in what was expected to be a “highly sensitive, complex and thorough investigation”.
“The death of such young children is a great shock to the whole community and this incident has had a profound and deep impact on all of us in the police,” she said.
“We’ll be making sure all those involved in the response and the subsequent investigation are given any welfare support they may need.”
St Edyth’s Church in Sea Mills has opened its doors to those affected by the tragedy, welcoming people from 08:30 GMT onwards.
“At a time of great sadness and disbelief, we’ve seen spontaneous acts of care and support,” said Ch Insp Hayward-Melen.
“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.”
The Independent Office for Police Conduct (IOPC) has been notified due to “prior police contact” earlier this month. The force said a mandatory formal referral will be made later.
In her statement, Ch Insp Hayward-Melen stressed that the deaths are believed to be an isolated incident, with no “ongoing risk to the wider community”.
She added that uniformed officers from the Neighbourhood Policing team will be in the area in the coming days, to provide reassurance and support to the local community.
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