This past year has been one of the FBI’s busiest — at least when it comes to terrorism cases. In the first 10 months of 2009, there have been possible plots exposed in Denver; Springfield, Ill.; Dallas; Boston; and, just this week, Chicago.

With so many alleged plots, it’s difficult to know how seriously to take any particular threat. So the intelligence community has an informal system for ranking them. The basic idea: The closer the link to al-Qaida, the more serious the plot.

Consider the case of Najibullah Zazi, the Denver-area shuttle bus driver who stands accused of plotting to blow up targets in New York City. Intelligence officials claim that the Zazi case is the most serious this country has faced since Sept. 11, 2001, because they believe Zazi had a direct link to senior al-Qaida leaders. Officials say that in addition to allegedly training in an al-Qaida camp, Zazi apparently called someone in Pakistan for instructions just before he was arrested.

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