A woman has been arrested in on suspicion of the murder of eight babies and the attempted murder of another six babies at a neonatal unit in Chester.
Police are investigating the deaths of a total of 17 babies at the Countess of Chester Hospital and 15 incidents in which infants collapsed but did not die.
On Tuesday investigating officers announced that a female healthcare professional was arrested for murder in relation to eight of the babies' deaths and on suspicion of attempted murder in relation to a further six babies.
The woman, who has not been identified, was being held in custody early on Tuesday, police said.
The arrest comes as part of a long-running police investigation following a high number of baby deaths at the hospital, initially between June 2015 and June 2016, when there were 15 fatalities.
Since the probe started the investigation widened and police are now looking into the deaths of 17 babies between March 2015 and July 2016.
Detective Inspector Paul Hughes, leading the investigation, described it as a "highly complex and very sensitive" police operation.
"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," he said.
“Due to the nature of the case and, as part of our ongoing enquiries, we have consulted with a number of medical specialists to ensure that we carry out as thorough an investigation as possible. We have also spoken to a large number of people to gather as much information as we can.”
He said the arrest was a “significant step forward” in the investigation but said: “This is an extremely difficult time for all the families and it is important to remember that, at the heart of this, there are a number of bereaved families seeking answers as to what happened to their children.”
“If anyone has any information that they want to pass onto the investigation team they are encouraged to get in touch via the Operation Hummingbird mailbox at operation.hummingbird@cheshire.pnn.police.uk.”
Police have not said if the arrested woman is a nurse, doctor or other health professional.
The hospital itself carried out a number of independent expert medical reviews into the deaths before calling in police.
Medical director Ian Harvey said: "We are continuing to support Cheshire Police with their ongoing investigation.
"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.
"The Countess is now equivalent to a Level 1 special care baby unit and we are 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.
