How Lucy Letby Was Caught: The Investigation Explained

Lucy Letby's final shift on the Countess of Chester Hospital's neonatal unit was on 30 June 2016. She wasn't arrested until 3 July 2018 — a gap of two years — and wasn't formally charged until November 2020, more than four years after she last worked on the ward. Understanding why that gap was so long, and what filled it, matters for anyone trying to make sense of a case that remains under active legal challenge.

This is the story of Operation Hummingbird: the Cheshire Police investigation that turned a hospital's internal suspicion into a criminal prosecution, built almost entirely on medical records, staffing data and documents recovered from Letby's own home, in the absence of any conventional crime scene or forensic trace evidence.

A Pattern Colleagues Noticed Before Police Did

By the time police became formally involved, hospital consultants had already spent roughly eighteen months trying to get management to act on their concerns, first raised informally from around October 2015 — covered in full in our timeline of the case. What they'd noticed wasn't a piece of physical evidence but a statistical pattern: Letby appeared to be present for a disproportionate number of the unit's sudden, unexplained collapses and deaths, at a level colleagues eventually judged too consistent to be coincidence. That pattern-recognition, done by clinicians rather than detectives, shaped the entire subsequent investigation.

March – May 2017: The Hospital Calls In Police

Hospital consultants asked management to involve police in March 2017, having been advised by a regional neonatal specialist that the pattern warranted formal investigation. A meeting between consultants and Cheshire Constabulary took place on 27 April 2017, and the trust publicly confirmed police involvement the following month, describing the move as necessary "to seek assurances that enable us to rule out unnatural causes of death." That announcement marked the formal start of Operation Hummingbird.

Building a Case Without a Crime Scene

Detectives faced an unusual challenge from the outset: no crime scene in the conventional sense, no weapon, no forensic trace evidence pointing to a specific individual. The entire case had to be built from thousands of pages of medical charts, nursing rotas, laboratory results and hospital correspondence, cross-referenced against staff shift patterns over an eighteen-month period. Investigators worked with independent medical experts to review each of the seventeen deaths and collapses under scrutiny individually, assessing whether natural clinical explanations could account for what had happened, before any specific allegation could be formulated with enough precision to support a charge.

This is a large part of why the investigation took years rather than months: building a prosecutable case required ruling out, incident by incident, the ordinary explanations a neonatal unit would expect among critically unwell, extremely premature babies — a population that carries its own substantial mortality risk independent of any external harm.

The First Arrest: 3 July 2018

Letby was arrested for the first time on 3 July 2018, at her home in Chester, on suspicion of murder and attempted murder. The search that followed produced material that would later feature heavily at trial: handwritten notes, a personal diary, and — perhaps most significantly for the shape of the prosecution case — more than 250 nursing handover sheets that should have remained at the hospital.

The Handwritten Notes

Among the material recovered was a green Post-it note found inside a 2016 diary, on which Letby had written "I don't deserve to live," alongside other lines including "I am evil I did this" and "I killed them on purpose because I'm not good enough to care for them." Prosecutor Nick Johnson KC invited jurors to read these lines "literally," as a confession.

The same notes also contained material the defence pointed to as running the other way: expressions of loneliness, declarations of affection for a colleague, and language reflecting general distress and self-blame rather than a specific admission of murder. The question of whether they amount to a confession or to the private writing of someone in acute psychological distress remains one of the most contested pieces of evidence in the case — a dispute that continues in the material now before the Criminal Cases Review Commission.

Two More Arrests, and a Widening Net

Letby was released on bail after her first arrest as the investigation continued. She was arrested a second time in June 2019 and released again, before a third and final arrest in November 2020, at which point she was formally charged. Between each arrest, investigators continued expanding the scope of Operation Hummingbird, including examining Letby's earlier training placements at other hospitals to establish whether the pattern extended further back into her career. Successive rounds of digital forensic examination of her phones, computers and social media activity also produced evidence used at trial, including that she had searched online for the families of babies she was accused of harming.

Building the Medical Case

Alongside the document-based evidence, prosecutors assembled an extensive body of expert medical opinion, principally from consultant paediatrician Dr Dewi Evans, arguing that specific collapses were consistent with air being deliberately introduced into babies' bloodstreams or nasogastric tubes, or with insulin poisoning identified through abnormal blood test results in two cases. This medical evidence took years to compile, partly because the underlying clinical presentations can also occur naturally, meaning each case had to be argued individually. It's also, more than any other single element of the case, what has come under the most sustained challenge since conviction — covered in full in our piece on the alleged methods.

November 2020: Charges Are Finally Brought

Letby was formally charged in November 2020 with seven counts of murder and fifteen counts of attempted murder, relating to seventeen babies. Nearly two more years passed before her trial opened at Manchester Crown Court in October 2022.

Why the Investigation Took So Long — and Why That Still Matters

Taken together, the gap between Letby's removal from the ward in mid-2016 and her trial in late 2022 — well over six years — reflects both the genuine complexity of building a case from medical statistics and documentary evidence rather than physical proof, and the scale of expert work required to translate a suspicious pattern into individually provable criminal charges. That same complexity is now central to the case's afterlife: the Criminal Cases Review Commission's review, still active as of August 2026, is working through much of the same underlying medical evidence Operation Hummingbird took years to assemble in the first place. A case built this slowly, and this technically, was always going to be difficult to either prove or challenge quickly.

Read the full timeline → · Browse DOUBT: The Case of Lucy Letby →