SAO PAULO, Brazil - Their faces hide behind scarves, gas masks and motorcycle helmets. TV images show them smashing cars, hurling bricks at police and setting tires ablaze on the streets of Sao Paulo and Rio de Janeiro. Brazilian politicians have claimed they are a national security threat. But who really are these protesters who plan to run riot at the World Cup as soon as the inaugural game kicks off on June 12? Brazil's police allege the militants form an extremist group called the Black Bloc, and say they are watching its leaders. The local press has published exposes...