Lady Justice Thirlwall's inquiry into the Countess of Chester Hospital found a "dysfunctional" management culture and an "us-versus-them" divide between staff that allowed nurse Lucy Letby to continue murdering and harming babies unchecked between June 2015 and June 2016. Staff who suspected wrongdoing stayed silent, those who raised concerns were dismissed, and hospital leaders opted for internal reviews rather than alerting police, allowing Letby's crimes to go undetected for far longer than they should have.

The inquiry examined over a million pages of documents and heard from more than 130 witnesses, concluding there was a "complete failure at all levels" to trigger safeguarding procedures, even once suspicion arose that Letby might be deliberately harming infants. Thirlwall said Letby should have been removed from the ward as a precaution while investigations took place, and singled out senior managers Alison Kelly, Ian Harvey and Tony Chambers for dismissing concerns as late as June 2016. The report also revealed earlier warning signs from Letby's training, including mixed performance feedback, a failed assessment, allegations of falsified medical records, and a close relationship with senior bosses that may have influenced their judgement. Letby is currently serving 15 whole-life sentences for murdering seven babies and attempting to murder seven others.