Cheshire Police are investigating a higher than expected number of baby deaths at the Countess of Chester Hospital.
It concerns deaths at it's neonatal unit between June 2015 and June 2016.
An independent review, which made 24 recommendations for improvements, also included a further detailed case note review by an independent neonatologist that has been unable to answer all of the questions regarding the cause of death for a number of babies.
Bosses say they have now asked for the input of Cheshire Police to 'seek assurances that enable us to rule out unnatural causes of death'.
Detective Chief Superintendent Nigel Wenham said: “Cheshire Constabulary has launched an investigation, which will focus on the deaths of eight babies that occurred between that period where medical practitioners have expressed concern.
“In addition the investigation will also conduct a review of a further seven baby deaths and six non-fatal collapses during the same period.
“We recognise that this investigation will have a significant impact on all of the families involved, staff and patients at the hospital and the public.
"Parents of the babies are being updated on the investigation and will be supported throughout the process by specially trained officers. We are committed to carrying out the investigation as quickly as possible.
“The investigation is in its very early stages. We are unable to provide any further details at this time.”
Countess of Chester Hospital Medical Director Ian Harvey said: “We are deeply sorry for the further distress and heartache this will cause. Throughout this we have never lost sight of the families left bereaved by the loss of their baby, and they will continue to be our main concern.
"At every point where the hospital has been able to share information with families and the public, we have done so.
"Approaching the police is not something we have undertaken lightly. This is to ensure we have been completely thorough in understanding what has happened here and to get the answers we and the families so desperately want.”
Specially trained officers have been in contact with those families directly affected.