Lucy Letby was convicted for the murder and attempted murder of more than a dozen babies. She has been called the worst child serial killer the UK had seen. But even before the trial was over experts had begun raising concerns about her conviction.

Then, last week, came a bombshell press conference in which a panel of renowned neonatal experts said they believed not just that Letby’s conviction was unsafe - but that there was no murder or deliberate harm. Instead they said the deaths had been caused by a series of factors including understaffing and a lack of skills on the ward to treat the babies they were caring for. So what is the evidence that the panel was looking at and why do so many questions seem to swirl around the Letby trial?

Josh Halliday, the Guardian’s North of England editor, reported on the trial and heard the evidence put before the jury. He said: “Until last week, I think I believed, on the balance of probabilities, that Lucy Letby was probably guilty.” However, the press conference raised some serious questions that have made him rethink his opinion.

The investigative reporter Felicity Lawrence has been examining the case for 18 months and tells Helen Pidd about what the ramifications of a retrial could be. “I think if it does turn out, as looks increasingly likely, to be a huge, historic miscarriage of justice, there’ll be questions that have to be answered by many institutions.”