The U.S. State Department issued its latest Mexico travel warning on May 5, 2015. As always, the Department of State's travel advisory errs on the side of caution, stressing the dangers to be aware of when traveling in Mexico as a foreign visitor.
It does not specifically mention Nogales (in particular the southern parts of the city) as a place to avoid for non-essential travel, as have previous travel warnings, although it mentions Nogales along with other major Sonoran tourist destinations in the context that Sonora is a "key region in the international drug and human trafficking trades" that can be "extremely dangerous for travelers."
In broad terms the warning mentions public gun battles that have occurred, kidnappings (some of which implicated the police), carjacking (which most frequently occurs at night on isolated roads) and highway robbery.
The advisory also describes the ongoing conflicts between government forces and criminal groups involved in drug trafficking, and that some Americans have been victims of the related violence. Still, the State Department admits that there is no evidence that organized criminal groups have targeted foreign visitors, and that "many of those" Americans affected by the violence were actually involved in criminal activity themselves.
And that is the story of the majority of violent crimes related to the battle between the government and the cartels in Mexico: The majority of deaths result from conflicts between the two groups, or between individuals from different cartels.
That does not mean that tourists cannot be innocent victims though, from being in the wrong place at the wrong time. For this reason the advisory repeats often-related advice for traveling in Mexico: avoid traveling at night; maintain a low profile and avoid wearing expensive jewelry; travel in groups when possible; and always be aware of your surroundings.
The tourist zone in Nogales is well patrolled and is generally not the scene of dangerous criminal activity. In fact, the City of Nogales makes great efforts to keep the area safe for visitors.
Following is the excerpt from the travel warning as it deals with travel in the State of Sonora:
Sonora: Nogales, Puerto Peñasco, Hermosillo, and San Carlos are major cities/travel destinations in Sonora - Sonora is a key region in the international drug and human trafficking trades and can be extremely dangerous for travelers. Travelers throughout Sonora are encouraged to limit travel to main roads during daylight hours. The region west of Nogales, east of Sonoyta, and from Caborca north, including the towns of Saric, Tubutama, and Altar, and the eastern edge of Sonora bordering Chihuahua, are known centers of illegal activity, and non-essential travel between these cities should be avoided. Travelers should also defer non-essential travel to the eastern edge of the state of Sonora, which borders the state of Chihuahua (all points along that border east of the northern city of Agua Prieta and the southern town of Alamos), and defer non-essential travel within the city of Ciudad Obregon and south of the city of Navojoa. You should exercise caution while transiting Vicam in southern Sonora due to roadblocks that can be instituted ad hoc by local indigenous and environmental groups. U.S. citizens visiting Puerto Peñasco should use the Lukeville, Arizona/Sonoyta, Sonora border crossing, and limit driving to daylight hours.