Airport Worker Screening: TSA Could Further Strengthen Its Approach to Estimating Costs and Feasibility of Security Measures, Report to Congressional Committees [open pdf - 2MB]
From the Highlights: "Any of the 1.8 million airport workers with unescorted access to security-restricted areas of the nation's airports could pose an 'insider threat.' The TSA Modernization Act (Act) required TSA to submit a study examining the cost and feasibility of enhanced worker screening measures. In its September 2020 study, TSA determined that implementing these measures is feasible and estimated implementation costs of $2.9-$3.6 billion, with ongoing annual costs of $2.5-$3.1 billion."
Government Accountability Office: http://www.gao.gov/