It was June 2016, and almost a year had passed since Stephen Brearey, the lead doctor at a neonatal unit in northwest England, first became concerned about a spate of troubling and unexpected deaths on his ward.

Five babies had died, and at least six others had experienced unusual complications. The neonatal ward at the Countess of Chester Hospital cared for premature and vulnerable babies, but the number of deaths was far above average for the unit. Something was desperately wrong.

Then, in the early evening of June 23, a baby boy — one of a set of newborn triplets — suddenly became sick and died. The following night, as the parents were still reeling, another of the triplets died.

The infants had been in the care of Lucy Letby, a seemingly conscientious and well-liked nurse. Dr. Brearey had noticed that she was present in every other suspicious case and raised that fact multiple times with executives, but he felt his concerns were dismissed.

After the second triplet died, he phoned a hospital executive and demanded that Ms. Letby be removed from the ward. The executive said there was no clear evidence against the nurse and insisted she was safe to work with, Dr. Brearey later told a court.

It would be another week before Ms. Letby, now considered the most prolific killer of children in modern British history, was moved to clerical duties, and months before the hospital’s senior managers contacted the police.

She was finally convicted last week of killing those boys by injecting air into their bodies, murdering five other babies and attempting to murder six others in her care.