From the U.S. State Department:
Millions of U.S. citizens safely visit Mexico each year for study, tourism, and business, including more than 150,000 who cross the border every day. The Mexican government makes a considerable effort to protect U.S. citizens and other visitors to major tourist destinations, and there is no evidence that Transnational Criminal Organizations (TCOs) have targeted U.S. visitors and residents based on their nationality. Resort areas and tourist destinations in Mexico generally do not see the levels of drug-related violence and crime reported in the border region and in areas along major trafficking routes.
...(C)rime and violence are serious problems throughout the country and can occur anywhere.
Tamaulipas: You should defer non-essential travel to the state of Tamaulipas. All USG employees are: prohibited from personal travel on Tamaulipas highways outside of Matamoros, Reynosa and Nuevo Laredo due to the risks posed by armed robbery and carjacking; may not frequent casinos and adult entertainment establishments within these cities; and in Matamoros are subject to a midnight to 6 a.m. curfew. Be aware of the risks posed by armed robbery and carjacking on state highways throughout Tamaulipas. In January 2011, a U.S. citizen was murdered in what appears to have been a failed carjacking attempt. While no highway routes through Tamaulipas are considered safe, many of the crimes reported to the U.S. Consulate General in Matamoros have taken place along the Matamoros-Tampico highway, particularly around San Fernando and the area north of Tampico.
Click the State Department link below for more information.
U.S. State Department Warning