Timeline of the investigation
- July 2015 – May 2017. Consultants repeatedly raise concerns with Trust executives; executives decline to contact police. See the September 2016 letter.
- May 2017. Operation Hummingbird opens. Eight months after the September 2016 consultants’ letter.
- May 2017 onwards. Dr Dewi Evans approaches Cheshire Police offering his expert services before he is formally instructed. The offer is accepted.
- 3 July 2018. Letby is arrested at her home in Chester for the first time.
- June 2019. Second arrest.
- November 2020. Third arrest and charge.
- October 2022 – August 2023. Original trial at Manchester Crown Court.
- June – July 2024. Child K retrial.
- July 2025. Three former senior executives of the Trust — separately from Letby — arrested on suspicion of gross negligence manslaughter. Investigation continues.
How the investigation was scoped
The police-side account comes from former Detective Chief Superintendent Nigel Wenham, the officer who led Operation Hummingbird. His Thirlwall Inquiry evidence is the principal public-record account. Independent commentary — including the anonymous Hummingbird whistleblower report released in December 2025 — argues the investigation’s scope was narrowed from day one in two specific ways:
- The briefing was nurse-focused. The material presented to police by the Trust and the consultants in 2017 treated the cluster as, in substance, a question of one nurse’s presence. Alternative explanations — the unit’s Level 2 designation, the staffing picture, the plumbing/sewage incidents — were not given equivalent prominence. The framing shaped the investigation.
- The expert instruction was unusual. Dr Dewi Evans approached the police offering his services, rather than being independently sourced. He became the lead causation expert for most counts. His methodology has since been comprehensively rejected by the 14-member Shoo Lee International Expert Panel.
What the CCRC is examining
The CCRC’s remit is conviction safety. But conviction safety depends, in part, on whether the evidence put before the jury was the best evidence available — or whether it was a selection shaped by investigatory framing. The arguments about Operation Hummingbird feed directly into that question.
The statistical argument (that the shift chart is a selection-bias artefact, see Statistics deep-dive) and the investigation-scoping argument are the same argument in different registers. Both say: the sample presented to the jury was not a random sample of the relevant universe of evidence; it was a pre-filtered sample.
The 2025 executives arrests
In July 2025, Cheshire Police — the same force that ran Operation Hummingbird — arrested three former senior executives of the Trust on suspicion of gross negligence manslaughter. No charging decisions have been announced. This second investigation is a significant development because it means the Trust’s own conduct is now itself being investigated for possible criminal liability for the deaths. That is not compatible with the earlier framing of those deaths as attributable to deliberate acts by one nurse.