" The U.S. economy is dependent on the expeditious flow of millions of tons of cargo each day. Cargo containers are an important instrument of global trade but also can present security concerns. CBP is responsible for administering container security programs, and its strategy for securing maritime cargo containers includes analyzing information to identify shipments that may contain terrorist weapons or other contraband. Because CBP has insufficient resources to examine every container, targeters use ATS to target which container shipments should be examined. GAO was asked to assess CBP's targeting efforts. This report addresses (1) how ATS supports CBP's targeting of maritime cargo container shipments for national security purposes and (2) the extent to which CBP assesses the effectiveness of ATS's national security targeting rules. GAO analyzed fiscal year 2011 CBP data on shipments and containers arriving at U.S. ports and containers scanned at these ports. GAO also visited six CBP units selected on the basis of the percentage of maritime shipments that were scored as high risk or medium risk for national security purposes at these locations in fiscal year 2011, among other factors. GAO also analyzed documents, such as CBP's ATS performance measures. "