Billions of pounds of cargo are transported on U.S. passenger flights annually. The Transportation Security Admin. (TSA) is the primary federal agency responsible for securing the air cargo system. The 9/11 Commission Act of 2007 mandated a system to screen 100% of cargo flown on passenger aircraft by Aug. 2010. This report reviewed TSA's progress in meeting the act's screening mandate, and any related challenges it faces for both domestic (cargo transported within and from the U.S.) and inbound cargo (cargo bound for the U.S.) The report reviewed TSA's policies and procedures, interviewed TSA officials and air cargo industry stakeholders, and conducted site visits at five U.S. airports, selected based on size, among other factors. Illus.