In his January 11 testimony to the Senate Select Committee on Intelligence, the FBI chief cited "a recent and rare" disclosure from MI-5 –the British Security Service–that "the United Kingdom is tracking 1,600 individuals who are part of at least 200 networks that are actively plotting terrorist attacks." By comparison, Mueller added, "in general, the number of subjects we are monitoring is proportional to the number of subjects BSS is monitoring–based on gross national population."

Well, that's not hard to figure out. America's population of 300 million is five times that of Britain's 60 million. Multiply Britain's 1,600 terrorism suspects by five and you get 8,000. FBI officials confirm that's a ballpark figure, but decline to put the figure on the record.

"You can draw your own conclusions," says one veteran agent.

One reason for the caution is that FBI officials see few operational cells in America and don't want the public thinking there are 8,000 terrorists running around. In the British disclosure, back in November, MI-5 chief Dame Eliza Manningham-Buller revealed that the Brits are following at least 30 plots, many using British-born operatives and tied to al Qaeda in Pakistan. By contrast, noted Mueller, "We see relatively fewer active plots involving physical attacks within the United States, less defined networks of extremists, and less developed attack planning compared to those described by the BSS director."

That's the good news–and helps explain why America hasn't had a major terrorist attack since 9/11. But then what are all those U.S. terrorism suspects up to? Lots of support activity, officials say. One FBI agent ticked off the list: raising money, recruiting, logistics, surveillance, communications. Still, a handful of operational cells are out there, investigators believe, and that's what they lose sleep over.

"It's like a hockey game," says a counterterrorism agent. "If you let one goal get scored, you've lost the game."