Customs and Border Protection officer Ramirez (L) checks the passports of pedestrians entering the United States at the San Ysidro border crossing from Mexico in San Ysidro, California January 31, 2008. U.S. border control began enforcement of a rule on Thursday that requires all U.S. and Canadian citizens above the age of 19 to present government-issued identification as proof of identity at all international land and sea border crossings. REUTERS/Fred Greaves (UNITED STATES)