Mexican federal police and army soldiers guard a U.S. Embassy vehicle after it came under attack by gunmen on a highway between Mexico City and Monterrey in February. A U.S. Immigration and Customs Enforcement agent was killed and another wounded in the attack. Many of Zetas' targets have been Mexican military and police personnel, but in recent years, U.S. law enforcement authorities also have come under attack. (Associated Press)

Mexican federal police and army soldiers guard a U.S. Embassy vehicle after it came under attack by gunmen on a highway between Mexico City and Monterrey in February. A U.S. Immigration and Customs Enforcement agent was killed and another wounded in the attack. Many of Zetas' targets have been Mexican military and police personnel, but in recent years, U.S. law enforcement authorities also have come under attack. (Associated Press)