Seven police officers and four gunmen were killed in a shootout in Mexico's northwestern state of Sinaloa, a stronghold of one of the country's ruthless drug gangs, officials said.
The state police officers were ambushed, but returned fire "and managed to kill four of the aggressors," an official with the Sinaloa state attorney general's office said.
Two agents were also wounded in the shootout, which took place near a tourist site known as El Fuerte, said the official, speaking on condition of anonymity.
Security forces launched a search for the assailants after the clash, with a police helicopter joining the hunt, the official said.
Sinaloa state is the stronghold of Joaquin "El Chapo" ("Shorty") Guzman, who the US Treasury Department considers "the world's most powerful drug trafficker." Forbes magazine estimates his wealth at $1 billion.
More than 50,000 people have been killed and 5,000 have gone missing in Mexico since outgoing President Felipe Calderon launched a military crackdown on the cartels when he took office in late 2006.
The spiraling drug-related violence has clouded Calderon's legacy, and left large areas of the country in a virtual state of war. It also will be the primary challenge facing Calderon's successor Enrique Pena Nieto.
The agents involved in Monday's gun battle were carrying out an operation in the northern Sinaloa town of Choix, which has been without a police chief since the June 29 murder of city chief Hector Echavarria, the official said.
"The aggressors are part of a cell that ... belongs to the Carrillo Fuentes-Beltran Leyva cartels," the official added.
The once powerful Ciudad Juarez-based Carrillo Fuentes group has been badly mauled in its turf war with the Sinaloa organization.
The Carrillo Fuentes organization saw its heyday in the late 1990s under the late Amado Carrillo Fuentes, the "Lord of the Skies," known for flying large planes loaded with cocaine into the United States.
The cartel run by the Beltran Leyva brothers was once one of Mexico's most powerful cartels and part of Guzman's Sinaloa Federation, but broke with "Chapo" and was subsequently destroyed following the arrest of its top leaders.