Good intelligence and cooperation among the United States and its allies enabled the interception of two package bombs dispatched from Yemen last week. U.S. and British officials reacted quickly to a last-minute tip from Saudi authorities identifying the packages, which had already traveled on at least four airplanes on their way to the United States. Neither the intelligence-sharing nor the swift response was likely to have happened a few years ago; what al-Qaida probably intended as a devastating election-eve strike instead has shown the improvement of defenses against it.

Still, the episode also revealed troubling vulnerabilities. The likely producer of the bombs, al-Qaida in the Arabian Peninsula, picked a transmission route that suffers from big security gaps. According to the Transportation Security Administration, nearly 40 percent of the cargo being delivered to the United States goes unscreened, and the TSA's goal for reaching 100 percent screening is 2013. Moreover, the bomb's sophisticated construction, which secreted an explosive known as PETN inside a printer's toner cartridge, made the device difficult to detect by some screening methods. Qatar Airways, which unknowingly transported one of the bombs on two passenger flights, said the explosives "could not be detected by X-ray screening or trained sniffer dogs." British authorities had trouble finding the second bomb even after the package containing it was identified.

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Members of Congress are calling for closing the cargo-screening loophole, but that is easier said than done: Congress had set an August 2010 deadline for screening all cargo on passenger planes, and it was not met. Vulnerabilities in cargo screening were identified in a September report by the Department of Homeland Security's inspector general. Its confidential recommendations should be implemented immediately.

Even then, however, the best defense against terrorist attacks will remain good intelligence and preventive action, and not focused only on attacks already underway: More must be done to go after al-Qaida's organization in Yemen. The United States is spending about $150 million to aid and train Yemeni forces and has dispatched Special Forces and drones to help in raids against al-Qaida targets. But the effort is small compared to the offensive against al-Qaida targets in Pakistan - and not enough is being done to help Yemen's weak government extend its authority to areas where al-Qaida is based.

President Obama said Friday that in addition to working to destroy the group in Yemen, "we'll also continue our efforts to strengthen a more stable, secure and prosperous Yemen so that terrorist groups do not have the time and space they need to plan attacks from within its borders." That is the only fool-proof defense.

- Washington Post