British intelligence and law enforcement officials have passed on a grim assessment to their U.S. counterparts, "It will be a miracle if there isn't a terror attack over the holidays in London," a senior American law enforcement official tells ABCNews.com. "It is not a matter of if there will be an attack, but how bad the attack will be," an intelligence official told ABCNews.com. Authorities say they are seeking at least 18 suspected suicide bombers.