Some U.S. law-enforcement officers based in London, NEWSWEEK has learned, have become extremely concerned about evidence regarding possible active Al Qaeda plots to attack targets in Britain. According to a U.S. government official, fears of terror attacks have prompted FBI agents based in the U.S. Embassy in London to avoid traveling on London's popular underground railway (or tube) system, which is used daily by millions of commuters. While embassy-based officers of the U.S. Secret Service, Immigration and Customs bureaus and the CIA still are believed to use the underground to go about their business, FBI agents have been known to turn up late to crosstown meetings because they insist on using taxis in London's traffic-choked business center.