I just received the below email from the US Embassy in Mexico:
Message to U.S. Citizens: Immigration Enforcement
U.S. citizens are reminded that if you enter Mexico by land and plan to travel beyond the immediate border area you must stop at a National Migration Institute (INM) office to obtain an entry permit (Forma Migratoria Multiple, or FMM), even if not explicitly directed to do so by Mexican officials.
When traveling in Mexico, the law requires that foreign visitors carry a passport and entry permit. You may be asked to present these documents at any point. If you do not present these documents, immigration authorities may lawfully detain you for up to 60 days while they review your immigration status.
Immigration check points are common in the interior of Mexico, including in popular tourist areas far from the border.
U.S. Citizens resident in Mexico should carry their resident identification cards at all times.
Actions to Take
• Obtain entry permit (FMM) at INM office if entering Mexico through a land border and traveling beyond the immediate border area.
• Carry your passport and entry permit (FMM) with you.
• For U.S. citizens who reside in Mexico, carry your resident identification card.
• Contact Form
• U.S. Embassy and Consulates in Mexico
• From Mexico: (55) 8526 2561
• From the United States: 1-844-528-6611