India, Feb. 24 -- The United States Department of State has issued a security alert for Americans in Mexico. The message is simple: shelter in place. Do not go outside. Wait for more instructions.

The warning comes after the killing of Mexico's most-wanted drug lord, Nemesio Oseguera Cervantes, known as "El Mencho." His cartel is fighting back. Highways are blocked. Vehicles are burning. At least 25 National Guard members are dead.

And the violence is spreading to places Americans love to visit.

Which Areas of Mexico Are Advised to Avoid? The State Department has named specific states and cities where Americans must take immediate precautions. Here is the full list.

Jalisco State This is ground zero. El Mencho was killed here. His car...