Detectives have been searching a nurse's home after a healthcare worker was held on suspicion of the murder of eight babies and the attempted murder of another six at a neonatal unit.
Officers are said to have arrived at the home of Lucy Letby, 28, early on Tuesday, hours before police announced a female "healthcare professional" had been arrested in a probe into the deaths of 17 infants at the Countess of Chester Hospital.
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Detectives said they are also looking at 15 non-fatal collapses at the unit between March 2015 and July 2016.
The arrest comes as part of a long-running investigation following a high number of baby deaths at the hospital.
Detective Inspector Paul Hughes, in charge of the investigation for Cheshire Police, said it represented a "significant step forward".
Police are said to have arrived at Ms Letby's home at around 6am, while officers were also seen at her parents' property in Hereford later on Tuesday.
A neighbour of the nurse in Blacon, Chester, said: "I saw lots of police cars arrive, a few cars then a big police van, and gradually more and more came and they put up the tent."
According to locals, officers moved a small, silver Suzuki car from the driveway of the address to erect the tent.
The car has an NHS parking permit in the window.
In a 2013 interview with the Chester And District Standard, Ms Letby said she cared for babies requiring various levels of support.
She had worked on the unit as a student nurse during three years of training before qualifying as a children's nurse at The University of Chester in 2011.
Ms Letby said she began working on the unit after graduating.
Police have not released the identity of the arrested health worker and did not say if she is a nurse, doctor or other health professional.
Detectives launched an investigation into infant deaths at the hospital in May last year, initially looking at the deaths of 15 babies between June 2015 and June 2016.
On Tuesday police said the probe had widened to include a total of 17 deaths and 15 non-fatal collapses of babies between March 2015 and July 2016.
DI Hughes said: "This is a highly complex and very sensitive investigation and, as you can appreciate, we need to ensure we do everything we possibly can to try to establish in detail what has led to these baby deaths and collapses.
"As a result of our ongoing inquiries, we have today arrested a healthcare professional in connection with the investigation.
"She was arrested earlier this morning on suspicion of murder in relation to eight of the babies and attempted murder in relation to six of the babies and is currently in custody.
"Whilst this is a significant step forward in our inquiries, it is important to remember that the investigation is very much active and ongoing at this stage."
The detective said the parents of all the babies were continuing to be kept updated and are being supported by specially trained officers.
The Countess of Chester Hospital had carried out a number of independent expert medical reviews into the deaths before calling in police.
Its medical director, Ian Harvey, said they were continuing to support police with their investigation.
He said: "Asking the police to look into this was not something we did lightly, but we need to do everything we can to understand what has happened here and get the answers we and the families so desperately want."
Mr Harvey added that the hospital is "confident the unit is safe to continue in its current form".
A report published in July 2016 by the Royal College of Paediatrics and Child Health (RCPCH) found significant gaps in medical and nursing rotas and insufficient staffing for the provision of longer-term, high-dependency and some intensive care.
The RCPCH report said paediatric consultants had been concerned since June 2015 about a higher than usual number of deaths on the ward, several of which were "unexplained" or "unexpected".
The hospital appeared as busy as ever on Tuesday with dozens of patients, staff and visitors coming and going from the vast site in Chester.
However security guards were on patrol and stationed near hospital entrances as a growing media pack arrived close to the main entrance.
Hospital officials said they will not be making any further statements.