The U.S. now inspects 4% of the 6 million shipments arriving at more than 100 ports; before the September 11, 2001, attacks that figure was 2%. About 20% of the cargo passes through overseas ports such as Hong Kong, where U.S. inspectors are being stationed. Cargo worth $1.2 trillion, or half of U.S. imports, arrives by sea. The rest comes in from Canada and Mexico. Given the sheer volume of arriving merchandise, there is a good chance that some shipments will include unwelcome items. But how can they be discoveredand identifiedwithout a lengthy inspection of every container?