Who Are Those Masked Men? The American Outlaws.
Don't worry.
You're not in the middle of a bank or train robbery. You haven't stepped onto the set of a Western movie remake. There hasn't been an outbreak of that SARS thing.
If you happen to see more than the normal amount of American Flag bandanas today (and by "normal amount" we do mean "zero") - you've just had the good fortune to cross paths with a member of the American Outlaws on a World Cup game day for the USMNT.