Mexican federal police guard a U.S. Embassy vehicle after it came under attack by unknown gunmen on Highway 57 between Mexico City and Monterrey, near the town of Santa Maria Del Rio, San Luis Potosi state, Mexico, Tuesday Feb. 15, 2011. A U.S. Immigration and Customs Enforcement agent was killed and another wounded in the attack.  (AP Photo/Pulso Diario de San Luis-Teodoro Blanco Vazquez)

Photo by: Teodoro Blanco Vazquez

Mexican federal police guard a U.S. Embassy vehicle after it came under attack by unknown gunmen on Highway 57 between Mexico City and Monterrey, near the town of Santa Maria Del Rio, San Luis Potosi state, Mexico, Tuesday Feb. 15, 2011. A U.S. Immigration and Customs Enforcement agent was killed and another wounded in the attack. (AP Photo/Pulso Diario de San Luis-Teodoro Blanco Vazquez)