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.
Sinaloa: You should defer non-essential travel to the state of Sinaloa except the city of Mazatlan, where you should exercise caution particularly late at night and in the early morning. One of Mexico's most powerful TCOs is based in the state of Sinaloa. With the exception of Ciudad Juarez, since 2006 more homicides have occurred in the state's capital city of Culiacan than in any other city in Mexico. Travel off the toll roads in remote areas of Sinaloa is especially dangerous and should be avoided. In the last year, the city of Mazatlan has experienced a level of violence (primarily confrontations between TCOs) not seen before and incidents of violence are occurring more frequently in tourist areas. ...
Click the State Department link below for more information.
U.S. State Department Warning