To security experts, the immense cargo ships that ferry more than 11 million containers into this country annually are potential Trojan horses-each one could easily harbor a WMD, such as a dirty bomb. Typically, only once the ships have been unloaded is their cargo subjected to random inspections and radiation scans. "There is an urgent need to effectively screen cargo before it reaches the ports," says Charles Meade, a senior scientist at the RAND Corporation, a nonprofit think tank.