Does incoming cargo pose a big security risk for the US?

The recent and tragic bomb explosions at the Boston Marathon should be a solemn reminder to all that terrorism is still a real danger. After 9/11 many people feared that the amount of incidents would be significantly higher that what they have been.

One reason for a relatively low occurrence in the US is the good work being done by port security. Trying to sneak explosives in cargo whether by ocean or air is difficult because of the high level of security detection at ports.

A variety of technology and methods are being used at ports to inspect for harmful products or devices. Potential threatening substances include chemical and biological substances, drugs, weapons and explosives. Of all these contraband, explosives may the most difficult to smuggle into the US. There is an abundance of explosive detection technology available and it is getting better. These methods include:
Dogs: Trained dogs can detect certain odors attributed to explosive devices. Certain breeds of dogs have very sensitive noses and have been successfully used in detection of drugs as well as explosive material
 Scent detection equipment: Highly sensitive detection devices can “sniff out” certain emissions common to materials used in explosives.
Radiation detection: Radioactive emissions can be detected by a variety of devices that are far more sensitive and accurate than the old fashion Geiger counter.
X-Ray Machines: Explosives can be detected by examining material densities and using computer analysis to compare density patterns with known explosive material composition.
A variety of other methods are currently being tested and marketed that are designed to make explosive device detection more foolproof and efficient.
One of the most common materials used for packing and crating is wood. From detection standpoint wood is a good choice of materials as it does not mask or hide some of the tell-tale emissions or radiations associated with most contraband. Cargo that is shipped in other types of material will get extra manual inspection including unpacking and visual inspection.