• Letby has been found guilty of murdering seven babies and attempting to murder six others in a neonatal unit at the Countess of Chester Hospital, in Cheshire.
• The first five murders all happened between June and October 2015 and - despite months of warnings - the final two were in June 2016.
Hospital bosses failed to investigate allegations against Lucy Letby and tried to silence doctors, the lead consultant at the neonatal unit where she worked has told the BBC.
The hospital also delayed calling the police despite months of warnings that the nurse may have been killing babies.
The unit's lead consultant Dr Stephen Brearey first raised concerns about Letby in October 2015.
No action was taken and she went on to attack five more babies, killing two.
Letby has been found guilty of murdering seven babies and attempting to murder six others in a neonatal unit at the Countess of Chester Hospital, in Cheshire.
The first five murders all happened between June and October 2015 and - despite months of warnings - the final two were in June 2016.
BBC Panorama and BBC News have been investigating how Letby was able to murder and harm so many babies for so long.
We spoke to the lead consultant in the unit - who first raised concerns about Letby - and also examined hospital documents. The investigation reveals a catalogue of failures and raises serious questions about how the hospital responded to the deaths.
Dr Brearey says he demanded Letby be taken off duty in June 2016, after the final two murders. Hospital management initially refused.
The BBC investigation also found:
- The hospital's top manager demanded the doctors write an apology to Letby and told them to stop making allegations against her
- Two consultants were ordered to attend mediation with Letby, even though they suspected she was killing babies
- When she was finally moved, Letby was assigned to the risk and patient safety office, where she had access to sensitive documents from the neonatal unit and was in close proximity to senior managers whose job it was to investigate her
- Deaths were not reported appropriately, which meant the high fatality rate could not be picked up by the wider NHS system, a manager who took over after the deaths has told the BBC
- As well as the seven murder convictions, Letby was on duty for another six baby deaths at the hospital - and the police have widened their investigation
- Two babies also died while Letby was working at Liverpool Women's Hospital
Before June 2015, there were about two or three baby deaths a year on the neonatal unit at the Countess of Chester Hospital. But in the summer of 2015, something unusual was happening.
In June alone, three babies died within the space of two weeks. The deaths were unexpected, so Dr Stephen Brearey, the lead consultant at the neonatal unit, called a meeting with the unit manager, Eirian Powell, and the hospital's director of nursing Alison Kelly.
"We tried to be as thorough as possible," Dr Brearey says. A staffing analysis revealed Lucy Letby had been on duty for all three deaths. "I think I can remember saying, 'Oh no, it can't be Lucy. Not nice Lucy,'" he says.
The three deaths seemed to have "nothing in common". Nobody, including Dr Brearey, suspected foul play.
But by October 2015, things had changed. Two more babies had died and Letby had been on shift for both of them.
By this point, Dr Brearey had become concerned Letby might be harming babies. He again contacted unit manager Eirian Powell, who didn't seem to share his concerns.
In an email, from October 2015, she described the association between Letby and the unexpected baby deaths as "unfortunate". "Each cause of death was different," she said, and the association with Letby was just a coincidence.
Senior managers didn't appear to be worried. In the same month - October 2015 - Dr Brearey says his concerns about Letby were relayed to director of nursing Alison Kelly. But he heard nothing back.
Dr Brearey's fellow consultants were also worried about Letby. And it wasn't just the unexpected deaths. Other babies were suffering non-fatal collapses, meaning they needed emergency resuscitation or help with breathing, with no apparent clinical explanation. Letby was always on duty.
In February 2016, another consultant, Dr Ravi Jayaram, says he saw Letby standing and watching when a baby - known as Baby K - seemed to have stopped breathing.
Dr Brearey contacted Alison Kelly and the hospital's medical director Ian Harvey to request an urgent meeting. In early March, he also wrote to Eirian Powell: "We still need to talk about Lucy".
Three months went by, and another two babies almost died, before Dr Brearey in May that year got the meeting with senior managers he had been asking for. "There could be no doubt about my concerns at that meeting," he says.
But others at the meeting appeared to be in denial. Dr Brearey said Mr Harvey and Ms Kelly listened passively as he explained his concerns about Letby. But she was allowed to continue working.
By early June, yet another baby had collapsed. Then, towards the end of the month, two of three premature triplets died unexpectedly within 24 hours of each other. Letby was on shift for both deaths.
After the death of the second triplet, Dr Brearey attended a meeting for traumatised staff.
He says while others seemed to be "crumbling before your eyes almost", Letby brushed off his suggestion that she must be tired or upset. "No, I'm back on shift tomorrow," she told him. "She was quite happy and confident to come into work," he says.
For Dr Brearey and his fellow consultants, the deaths of the two triplets were a tipping point. That evening, Dr Brearey says he called duty executive Karen Rees and demanded Letby be taken off duty. She refused.
Dr Brearey says he challenged her about whether she was making this decision against the wishes of seven consultant paediatricians - and asked if she would take responsibility for anything that might happen to other babies the next day. He says Ms Rees replied "yes".
The following day, another baby - known as Baby Q - almost died, again while Letby was on duty. The nurse still worked another three shifts before she was finally removed from the neonatal unit - more than a year after the first incident.
The suspicious deaths and collapses then stopped.
Letby still wasn't suspended, however.
Instead, she was moved to the hospital's risk and patient safety office. Here she is believed to have had access to sensitive documents relating to the hospital's neonatal unit. She also had access to some of the senior managers whose job it was to investigate her.
On 29 June 2016, one of the consultants sent an email under the subject line: "Should we refer ourselves to external investigation?"
"I believe we need help from outside agencies," he wrote. "And the only agency who can investigate all of us, I believe, is the police."