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.

Nevertheless, U.S. travelers should be aware that the Mexican government has been engaged in an extensive effort to counter TCOs which engage in narcotics trafficking and other unlawful activities throughout Mexico. The TCOs themselves are engaged in a violent struggle to control drug trafficking routes and other criminal activity. As a result, crime and violence are serious problems throughout the country and can occur anywhere. U.S. citizens have fallen victim to TCO activity, including homicide, gun battles, kidnapping, carjacking and highway robbery.

Nuevo Leon: You should defer non-essential travel to the state of Nuevo Leon, except the metropolitan area of Monterrey, where you should exercise caution. The level of violence and insecurity in Monterrey has increased, illustrated by an attack on a popular local casino in August that resulted in 52 deaths. One U.S. citizen was injured in that incident. Local police and private patrols do not have the capacity to deter criminal elements or respond effectively to security incidents. As a result of a Department of State assessment of the overall security situation, on September 10, 2010, the Consulate General in Monterrey became a partially unaccompanied post with no minor dependents of USG personnel permitted. ...

Please see the State Department link below for more information.

U.S. State Department Warning