Three managers at the hospital where Lucy Letby worked have been arrested on suspicion of gross negligence manslaughter.
They were in senior roles at the Countess of Chester Hospital in 2015 and 2016 and have been bailed pending further enquiries, Cheshire Constabulary said. Their names have not been made public.
Letby, 35, was found guilty of murdering seven children and attempting to murder seven more between June 2015 and June 2016 while working in the hospital's neonatal unit.
Detective Superintendent Paul Hughes explained that gross negligent manslaughter focuses on the "action or inaction of individuals".
There is also an investigation into corporate manslaughter at the hospital, which began in October 2023.
That focuses on "senior leadership and their decision-making", Mr Hughes said. The intention there is to determine whether any "criminality has taken place concerning the response to the increased levels of fatalities".
The scope was widened to include gross negligence manslaughter in March of this year.
Mr Hughes said it is "important to note" that this latest development "does not impact on the convictions of Lucy Letby for multiple offences of murder and attempted murder".
He added: "Both the corporate manslaughter and gross negligence manslaughter elements of the investigation are continuing and there are no set timescales for these.
"Our investigation into the deaths and non-fatal collapses of babies at the neo-natal units of both the Countess of Chester Hospital and the Liverpool Women's Hospital between the period of 2012 to 2016 is also ongoing."
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Earlier this year, lawyers for Lucy Letby called for the suspension of the inquiry into the events surrounding her crimes, claiming there was "overwhelming and compelling evidence" that her convictions were unsafe.
In February, an international panel of neonatologists and paediatric specialists told reporters that poor medical care and natural causes were the reasons for the collapses and deaths.
Their evidence has been passed to the Criminal Cases Review Commission (CCRC), which investigates potential miscarriages of justice, and Letby's legal team hopes her case will be referred back to the Court of Appeal.
(c) Sky News 2025: Three members of Lucy Letby hospital's senior leadership team arrested
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