"Billions of pounds of cargo are transported on U.S. passenger flights annually. The Department of Homeland Security's (DHS) Transportation Security Administration (TSA) is the primary federal agency responsible for securing the air cargo system. The 9/11 Commission Act of 2007 mandated DHS to establish a system to screen 100 percent of cargo flown on passenger aircraft by August 2010. As requested, GAO 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 United States) and inbound cargo (cargo bound for the United States). GAO 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"--Highlights.