BACKGROUND: Serious incident (SI) investigations aim to identify factors that caused or could have caused serious patient harm. This study aimed to use the Human Factors Analysis and Classification System (HFACS) to characterise the contributory factors identified in SI investigation reports. METHODS: We performed a content analysis of 126 investigation reports from a multi-site NHS trust. We used a HFACS-based framework that was modified through inductive analysis of the data. RESULTS: Using the modified HFACS framework, 'unsafe actions' were the most commonly identified hierarchical level of contributory factors in investigation reports, which were identified 282 times across 99 (79%) incidents. 'Preconditions to unsafe acts' (identified 223 times in 91 (72%) incidents) included miscommunication and environmental factors. Supervisory factors were identified 73 times across 40 (31%) incidents, and organisational factors 115 times across 59 (47%) incidents. We identified 'extra-organisational factors' as a new HFACS level, though it was infrequently described. CONCLUSIONS: Analysis of SI investigation reports using a modified HFACS framework allows important insights into what investigators view as contributory factors. We found an emphasis on human error but little engagement with why it occurs. Better investigations will require independence and professionalisation of investigators, human factors expertise, and a systems approach.