This article focuses on the control of international mobility through the gathering, processing, and sharing of air travellers’ data. While a lot has been written about pre-emptive rationalities of security translated into the functionalities of IT systems used for border controls, we take a step further and investigate how these rationalities are operationalised through data transfer, screening, validation, discarding, profiling, contextualisation, calibration, and adjustment practices. These practices may seem banal and technical; however, we demonstrate how they matter politically as they underpin the making of international security. We do so by analysing the work of Passenger Information Units (PIUs) and retracing how they turn Passenger Name Record (PNR) data into actionable intelligence for counterterrorism and the fight against serious crime. To better understand the work of PIUs, we introduce and unpack the concept of ‘epistemic fusion’. This explicates how security intelligence comes into being through practices that pertain to cross-domain data frictions, the contextualisation of data-driven knowledge through its synthesis with more traditional forms of investigatory knowledge and expertise, and the adjustment of the intelligence produced to make it actionable on the ground.