A "healthcare professional" has been arrested on suspicion of murdering eight babies and attempting to kill six others at a hospital.
Cheshire Police said the woman's arrest was a "significant step forward" in an investigation into deaths at the Countess of Chester Hospital.
Officers have been investigating the deaths of 17 newborns at the neonatal unit between March 2015 and July 2016.
A spokesman said the probe also covered 15 "non-fatal collapses".
Det Insp Paul Hughes said the investigation was "highly complex and very sensitive" and officers were doing "everything we possibly can to try to establish in detail what has led to these baby deaths and collapses".
He added there were "no set timescales for this coming to a conclusion but we remain committed to carrying out a thorough investigation as soon as possible".
Hospital medical director Ian Harvey said asking police to investigate the deaths had not been "something we did lightly, but we need to do everything we can to understand what has happened".
He added the neonatal unit was "safe to continue in its current form" and remained open to women over 32 weeks in their pregnancy.