Lucy Letby has been questioned over her time working at Liverpool Women’s Hospital.
Police began investigating the former nurse’s background after she was was jailed for the murder of seven babies in the Countess of Chester neonatal unit
She was also convicted for the attempted murder of a further six infants at the facility during a 12-month period between June 2015 and June 2016.
Detectives have now explored the 34-year-old’s time on placement at the Women’s in 2012 and 2015, the latter just months before her first killing.
An inquiry heard that babies’ breathing tubes became dislodged in 40% of Letby’s shifts between October-December 2012 and January-February 2015.
Letby, who is serving 15 full life sentences, was interviewed under caution as part of the investigation into deaths and non-fatal collapses during that period.
Cheshire Police confirmed that the Hereford native also agreed to be questioned in prison about similar instances at the Countess of Chester.
The care of around 4,000 babies admitted to hospital during Letby’s time as a neonatal nurse are being reviewed by detectives as part of the investigation.
Letby is currently serving her life sentence at HMP Bronzefield, a category A female prison in Surrey.