After the 9/11 terrorist attacks, the Department of Homeland Security (DHS) was formed to secure our nation against the threat of another large attack. DHS, through the United States Customs and Border Patrol service, maintains a presence at U.S. borders to screen, inspect, and interdict suspicious cargo to identify and eliminate contraband entering the United States. Since 2002, RPMs have become a powerful tool in the DHS arsenal. RPMs examine every vehicle or package that moves through them, checking for the presence of radioactive materials. The Department of Energy and Department of Defense also use RPMs to detect and interdict materials of proliferation concern at foreign ports and military bases.