This cross-sectional study utilises an explanatory-sequential, mixed-methods design, using quantitative surveys (N = 275) and semi-structured interviews (N = 17) to explore the psychological impacts of investigation type on a sample of UK police senior investigating officers (SIOs). Prevalence rates of post-traumatic stress disorder (PTSD) (5.4%) and complex post-traumatic stress disorder (CPTSD) (4.1%) are also established. Analyses demonstrate that organisational stress (B = 1.15), experiencing trauma 6 to 12 months ago (B = −1.72) and child homicide investigations (B = 1.76) predict PTSD, whereas operational stress (B = 1.82), experiencing trauma 6 to 12 months ago (B = −1.45) and 10 to 20 years ago (B = 3.92) predict CPTSD. Child homicide is the only investigation type to predict all three PTSD symptoms, while adult homicide predicts lower re-experiencing (B = −1.50) and sense of threat (B = −1.07). The qualitative analysis suggests that although SIOs may become more resilient to traumatic experiences through coping processes of avoidance and detachment, this career-long experience of trauma exposure leads to psychological vulnerability. Supporting the quantitative findings, increased PTSD resulting from child homicide investigations appears to be related to levels of personal resonance. Many SIOs that maintained it is not any particular type of investigation, however. It is thus argued that the relationship between child homicide and PTSD is influenced by Lazarus and Folkman's concept of ‘centrality’. The practical findings of this paper are discussed, and implications for police practice and supporting officer well-being moving forward are considered.