With the passage of ATSA in November 2001, TSA assumed from the Federal Aviation Administration (FAA) responsibility for securing the nation's civil aviation system. In accordance with ATSA, TSA is responsible for the procurement, installation, and maintenance of explosive detection systems, including EDS and ETD, used to screen checked baggage for explosives (see figs. 1 and 2) at TSA-regulated airports.11 11 "TSA-regulated airports" refers to all airports that implement TSA-approved security programs pursuant to 49 C.F.R. part 1542, which includes airports regularly serving air carrier operations regulated pursuant to 49 C.F.R. 1544.101 and 1546.101 (what may be characterized as U.S and foreign-flagged commercial air carriers), and at which TSA performs or oversees the performance of screening activities. EDS machines identify suspicious items or anomalies in checked baggage that could indicate the presence of explosives or detonation devices. At airports with EDS, EDS machines are generally employed for primary screening of checked baggage while ETD machines are used for secondary screening to help resolve questions raised by EDS screening. At airports without EDS, ETD machines are used as the primary method for screening checked baggage."