Lucy Letby likely murdered or attacked more children than is currently known, a doctor has told an inquiry.
The 34-year-old neonatal nurse from Hereford is serving 15 whole-life orders after she was convicted at Manchester Crown Court of murdering seven babies and attempting to murder six others.
This year she was also found guilty of attempting to murder another baby.
Please use Chrome browser for a more accessible video player
The crimes took place between June 2015 and June 2016 at the Countess of Chester Hospital.
Giving evidence to the Thirlwall Inquiry, which is looking into the events surrounding her crimes, neonatal clinical lead Dr Stephen Brearey said: "I think it's likely that Letby didn't start becoming a killer in June 2015, or didn't start harming babies in June 2015."
Representing the families of Letby's victims at the inquiry, Peter Skelton KC asked Dr Brearey: "Child A was murdered on 8 June 2015. Is it your view that she [Letby] had murdered or assaulted children in your hospital prior to that date?"
The doctor replied: "I think that's likely, yes."
He agreed that "on reflection" a number of unexpected collapses and deaths prior to June 2015 now "appear suspicious".
However, he said that at the time he didn't have concerns about those incidents, adding: "We just thought we were going through a busy or particularly difficult patch."
Dr Brearey continued: "Our perception of what is normal for a neonatal unit in terms of the number of collapses you might expect in a week, a month, or a year, that changed."
Mr Skelton said: "One of the effects of normalising the abnormal is that when abnormalities occur you don't recognise it and you don't take appropriate action - is that a reflection that resonates with you?"
Dr Brearey agreed.
Please use Chrome browser for a more accessible video player
'Not nice Lucy'
After three deaths in June 2015, when Dr Brearey was told Letby was identified as the one member of staff present on each occasion, he recalled his first reaction was to say: "Oh no, not Lucy. Not nice Lucy."
Counsel to the inquiry Rachel Langdale KC asked: "Why 'oh no, not nice Lucy'? What was the point of the 'oh no', what was the link being made in your mind?"
Dr Brearey said: "Well obviously, some part of my mind was thinking the worst."
Ms Langdale asked: "What did your mind jump to?"
Dr Brearey replied: "The concern that there might be someone harming babies."
Follow Sky News on WhatsApp
Keep up with all the latest news from the UK and around the world by following Sky News
'Set a few alarm bells going'
In July 2016, Letby was moved to clerical duties after consultants, including Dr Brearey, raised their concerns with executives.
However, Cheshire Police were not called until May 2017 and there had been plans shortly before to return Letby to the unit.
Dr Brearey said that the death of a baby girl, referred to during court proceedings as Baby I, was a "significant moment that raised my level of concern quite considerably".
The medic said the infant's death had "set a few alarm bells going".
Read more from Sky News:
'Increasing risk' of tropical infections
Snow and ice hits large parts of UK
Is Putin ready to hit the nuclear button?
The inquiry previously heard that the dislodgement of breathing tubes, which was how Letby tried to kill Child K, generally occurs on less than 1% of shifts.
However, it happened on 40% of shifts that Letby worked when she was a trainee at Liverpool Women's Hospital.
Police are investigating Letby's time here, between October and December 2012, and January and February 2015.
The inquiry at Liverpool Town Hall is expected to sit until early 2025, with its findings published by late autumn next year.