Port and Maritime Security: Background and Issues for Congress [Updated May 27, 2005] [open pdf - 116KB]
"The U.S. maritime system consists of more than 300 sea and river ports with more than 3,700 cargo and passenger terminals. However, a large fraction of maritime cargo is concentrated at a few major ports. Most ships calling at U.S. ports are foreign owned with foreign crews. Container ships have been the focus of much of the attention on seaport security because they are seen as vulnerable to terrorist infiltration. More than 9 million marine containers enter U.S. ports each year. While the Bureau of Customs and Border Protection (CBP) analyzes cargo and other information to target specific shipments for closer inspection, it physically inspects only a small fraction of the containers."
CRS Report for Congress, RL31733