The revelation that the FBI planted a spy in a Southern California mosque was explosive news in a Muslim community that has long suspected the government of even broader surveillance. Muslim-American organizations have demanded an inquiry. Some say the news has rattled their faith in American democracy. Despite the reaction, former FBI agents and federal prosecutors say spying on mosques is still one of the government's best weapons to thwart terrorists and that the benefit to national security is likely to far outweigh any embarrassment to the agency. "What matters to the FBI is preventing a massive attack that might be planned by some people ... using the mosque or church as a shield because they believe they're safe there," said Robert Blitzer, the FBI's former counter-terrorism chief.