The documentary makers had exclusive access to the key investigators at Cheshire Police, who spent years building the case against Letby.
The producers also had bodycam footage of the neonatal nurse’s three arrests and police interviews.
They also spoke to Dr John Gibbs, one of the consultant paediatricians who first raised concerns about Letby with hospital management.
He remembered Letby as being “quiet”, adding nothing about her had stood out.
But as the deaths continued, the neonatal consultants had noticed she was the one person who seemed to be on duty.
Gibbs said they raised their concerns on a number of occasions, but others defended her work.
When she was transferred from working nights to day shifts, he said the timings of the deaths and non-fatal collapses also changed.
Gibbs said: “At that stage, we as a consultant body realised that we could not delay any further. Lucy Letby had to be removed from the neonatal unit.”
Eventually the police were brought in and Letby was charged in 2020.
Sarah said she knew attending the subsequent trial would be challenging.
She said she also “knew Lucy Letby was going to take the stand, so I needed to face her”.
Sarah told the documentary: “I sat three metres away from her. She looked at me a dozen times, staring. Every time she looked at me I’d have to look down.”
Letby was eventually found guilty of murdering seven babies, including Baby D, and attempting to murder seven more.
In the years since her convictions, though, there has been a growing campaign questioning their safety.
The documentary also interviews Letby’s barrister Mark MacDonald.
Brought in to represent her in late 2024, he is convinced of her innocence.
“No-one saw her do anything wrong,” he said. “There is no CCTV. There is no motive – none.”