The al-Qaida of Sept. 11, 2001, has been greatly degraded. But new, like-minded groups are becoming growing threats. And people who live in the U.S. are not immune to their messages. Dozens of Americans, inspired by al-Qaida's ideology, are known to have plotted to kill innocent people inside the U.S. and abroad. There is a fine line between expressing one's opinion, however hateful, and being motivated to commit violence. The government is constantly trying to identify the latter.