LONDON -- A woman has been arrested on suspicion of killing eight babies and attempting to kill another six while working as a healthcare professional at the Countess of Chester Hospital in Cheshire, North Wales.

A police investigation launched more than a year ago originally was looking into the deaths of 15 newborn babies between 2015 and 2016 but has since been expanded to probe 17 deaths and a further 15 babies who suffered non-fatal collapses.

Neither the police nor the hospital would confirm the exact role of the healthcare worker, but police said they were searching a property a mile away.

Detective Inspector Paul Hughes, who is in charge of the investigation, said: "Since it was first launched in May 2017, a dedicated team of detectives have been working tirelessly on this case. 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."

Speaking about the arrest, he added: "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. There are no set timescales for this coming to a conclusion but we remain committed to carrying out a thorough investigation as soon as possible."

The Countess of Chester Hospital was found in a 2017 study by the Mothers and Babies: Reducing Risk through Audits and Confidential Enquiries to be have a neonatal death rate 10 percent higher than that found in similar units at other hospitals.

The hospital told ABC news that it had no comment other than the statement it issued on its website, which 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."

The last major case involving infant deaths was the 1991 murders committed by Beverly Allitt, a nurse who killed four children by lethal injection and who attempted to kill three more.