Islam teaches peace, they told anyone who would listen in news conferences, at interfaith services and, most famously, standing in a mosque with President Bush. But five years later, the target audience for their pleas has shifted. Now the faith's American leaders are starting to warn fellow Muslims about a threat from within. The 2005 subway attacks in London that investigators say were committed by British-born and -raised Muslims, and the relentless Muslim-engineered sectarian assaults on Iraqi civilians, are among the events that have convinced some U.S. Muslims to change focus. "This sentiment of denial, that sort of came as...