Incidents & Accidents · Mandatory · Construction

Accident Investigation Report

A structured investigation into the root causes of an accident or incident. It uses methods such as the 5 Whys or fault tree analysis to identify underlying failures and recommend corrective actions to prevent recurrence.

Last reviewed: 29 March 2026 — This guide reflects UK law as of this date. MHSWR 1999 remains current with no amendments enacted as of 29 March 2026. Next scheduled review: 29 March 2027.

1. Accident Investigation — Finding Root Causes, Not Just Blame

The purpose of an accident investigation is not to assign blame — it is to understand what happened and why so that it cannot happen again. HSG245 is explicit: 'Be wary of blaming individuals.' An investigation that concludes 'the worker wasn't paying attention' has found the immediate cause and stopped. It must ask: why wasn't the worker paying attention? What about the system, the environment, or management allowed a moment's inattention to lead to serious injury?

The 5 Whys technique moves from immediate cause to root cause. Each 'Why?' peels back a layer, typically revealing management and system failures that, once addressed, prevent recurrence far more effectively than blaming the individual involved.

Start the investigation immediately — evidence and witness recall are freshest in the first hours

HSG245 emphasises beginning the investigation as soon as practicable, ideally the same day. Evidence deteriorates rapidly: equipment is moved, conditions change, and weather alters the scene. Witnesses' recollections change as they discuss events with colleagues. A witness statement taken at 4pm on the day of the incident is significantly more reliable than one taken the following Monday.

2. Accident Investigation Report — Structure

Section	Content
Executive summary	Brief description. Injury sustained. RIDDOR status. Key finding. Principal recommendation.
Background	Work activity at the time. Site conditions. Equipment and materials involved.
Chronology	Timeline of events before, during, and immediately after the incident.
Immediate cause	What directly caused the injury or damage.
Underlying causes	Why was the hazard present? What control failed? What procedure was not followed?
Root causes	Management and system failures that allowed the hazard to exist uncontrolled. Identified using 5 Whys.
Risk control measures	Engineering controls, procedural changes, training, and supervision improvements.
Actions	Specific actions, named responsible persons, and deadlines.
Sign-off	Lead investigator and senior management. Date actions completed and verified.

3. Common Mistakes

Identifies only the immediate cause and stops

‘Worker slipped on a wet surface’ is the starting point, not the endpoint. Why was the surface wet? Why was it not signed? Why was there no procedure for wet conditions? Each ‘Why?’ moves toward the root cause and the corrective actions that will actually prevent recurrence.

Conducted by persons with an interest in the outcome

A site manager who was responsible for the area where the incident occurred has an inherent bias. For significant incidents, an independent investigator — an H&S advisor, external consultant, or senior colleague from outside the site — provides the objectivity required for a credible investigation.

Actions identified but no tracking to completion

Actions identified and assigned but never tracked or closed defeats the entire purpose of the investigation. The report must include a follow-up review date, confirmation from the responsible person, and verification that the control is effective.

Not interviewing all witnesses

Different witnesses observe different aspects of an incident. The injured worker may not remember the sequence. The supervisor may have noticed the hazard beforehand. A colleague nearby may have seen the full sequence clearly. Each witness should be interviewed individually, not as a group.

Conclusions not communicated to the workforce

Findings and corrective actions must be shared with the workforce. An investigation that remains in the management office and is never shared misses the most important opportunity for learning. Toolbox talks are the most effective way to communicate investigation findings and demonstrate that lessons are being acted upon.

4. Frequently Asked Questions

What is the 5 Whys technique?▾

An iterative questioning method — for each cause, ask ‘Why?’ until the answers become system or management failures (typically 5–7 iterations). Example: a worker was burned by hot liquid. Why? A pipe fitting failed. Why? The wrong type was used for the temperature. Why? The operative selected it without checking the specification. Why? No specification document was available in the maintenance room. Why? Specifications were stored only in the main office. Root cause: inadequate availability of technical documentation at point of use.

What is the difference between investigating an accident and a near miss?▾

The process is the same — gather evidence, identify causes, implement controls. The difference is that an accident has consequences providing clear information about what failed, while a near miss involves the same failure but consequences were avoided by chance. A near miss with potential fatality deserves the same investigation resource as an actual fatality.

When should an external specialist be used?▾

In cases of fatality or life-changing injury; where technical complexity exceeds site expertise; where there is risk of internal compromise; during concurrent HSE investigation; or where legal and insurance implications are significant. External specialists such as forensic engineers, H&S consultants, and occupational health physicians provide both expertise and independence.

Does the investigation report need to be shared with HSE?▾

It is not automatically required, but HSE has statutory powers to require production of documents under HSWA 1974. The report should be approached as if it will be read by HSE and the courts — thorough, factual, and honest. Investigation reports are potentially admissible in both criminal and civil proceedings.

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