Gunmen hung a man from a pedestrian bridge over a busy avenue in the Mexican city of Monterrey Tuesday and shot him to death in front of dozens of motorists.

A police investigator in Nuevo Leon state, where Monterrey is located, said the man was alive when he was hung and died after being shot.

The investigator, who spoke on condition of anonymity because he was not authorized to discuss the case, said the assailants left a threatening message but he wouldn't reveal what it said.

Monterrey's overpasses have seen several gang killings this year, often carried out in daylight in view of drivers on busy streets below. The Gulf and Zetas drug cartels are fighting for control of the city.

Also Tuesday in the Pacific coast resort of Acapulco, two teenage boys of about 15 years of age were shot to death by unidentified assailants. State police in Guerrero state, where Acapulco is located, said gunmen chased the teenagers down and shot them with about a dozen 9 mm rounds, the type of machine-pistol ammunition favored by drug gangs.

Police also found a man's dismembered body in a car in another Acapulco neighborhood.

As drug-related violence continued, Mexico's top police official, Public Safety Secretary Genaro Garcia Luna, met in Mexico City Tuesday with the head of the U.S. Drug Enforcement Administration, Michele Leonhart, to review the results of bilateral cooperation in the fight against drug cartels.

The two officials toured Mexico's federal police intelligence center, and discussed information sharing, weapons and drug trafficking.

A statement by Garcia Luna's office said bilateral cooperation "has enabled us to achieve success in the fight against criminal gangs."