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.
Read on
- The anonymous 150-page whistleblower report
- DCS Wenham’s Thirlwall evidence
- Dr Dewi Evans methodology — evidence page
- Statistics deep-dive
The May 2017 opening: scoping decisions
Operation Hummingbird opened in May 2017, eight months after the consultants’ September 2016 joint letter to the Trust executive team asking for police referral. The opening followed the Trust’s decision to commission the RCPCH service review (November 2016) instead of forensic investigation; the RCPCH report itself recommended independent forensic investigation, which was not acted on. The eight-month delay is one of the principal institutional-failure findings the Thirlwall Inquiry will address.
The investigative scoping decisions taken at the opening — which deaths to treat as potentially suspicious, which experts to instruct, which material to retain and test forensically, which to treat as peripheral — shaped the eventual 2020 charging decision and the 2022-2023 trial. Independent analysts have argued the investigation adopted a suspect-first orientation from early in its scoping; the December 2025 Hummingbird whistleblower report addresses this specifically.
The Dewi Evans self-referral
Dr Dewi Evans self-referred to Cheshire Police in 2017 offering his services as a neonatal expert witness. The decision to instruct him was taken at SIO level. The full correspondence trail between Evans and the Hummingbird investigation has not been made fully public; parts have surfaced through Thirlwall document discovery. The self-referral-rather-than-blinded-instruction pattern is one of the structural features of the investigative-scoping critique.
UK criminal-evidence practice generally expects expert witnesses to be instructed through neutral processes, not self-referred. Dr Evans’s self-referral and the SIO-level decision to instruct him is one of the data points the post-conviction expert-instruction-standards critique engages.
The seven-year investigation timeline
The investigation timeline from May 2017 opening to August 2023 verdicts spans seven years. The protracted timeline reflects the complexity of the medical-evidence assembly, the size of the candidate-cohort review, and the sequencing of arrests (2018, 2019, 2020) and charging (November 2020). The extended timeline is itself relevant to memory-science questions: witness evidence at trial in 2022-2023 covered events from 2015-2016, and the 2024 Child K retrial covered events nine years old at that point.
The whistleblower-report structural claim
The December 2025 anonymous Hummingbird whistleblower report (hosted on lucyletby.org) sets out a structured account of the investigation as suspect-first from its outset. Its specifically-named allegations include the Brearey mortality-rate misrepresentation claim and the Sir Duncan Nichol early Allitt-framing claim. The report is anonymous and its claims are not independently verified, but its structural framing aligns with the documentary picture independent commentators have assembled from public sources.