The attack has added to a sense of alarm in an area that has long been the heart of Mexico's tourism industry, and the violence hints at the criminal presence in the region.

Authorities in Cancun declared a "code red" after at least 10 motorcycle-borne gunmen assaulted the state prosecutor's office and the

That shooting left five people dead — including the head of security at the club, who was reportedly the target, and an American woman who was killed in the stampede of people escaping the scene. Another 15 people were wounded, several of them by bullets fired by security personnel and others on the scene.

Officials initially said that attack was the result of a personal dispute, but in the hours after, drug-cartel conflict emerged as potential motive.

The possibility of cartel involvement was strengthened by the appearance of four narcomantas — banners or signs typically left at the scene of cartel-related crimes — on Tuesday.

They were allegedly signed by the "Old School Zetas," a faction of the fragmented Los Zetas cartel, and made threats against rival criminal groups in the area.