Mutilated bodies of nine people turned up in the southern state of Guerrero, authorities said Friday, amid an increase in drug-related violence there.

The bodies had been beheaded, hacked to pieces and left in 18 black plastic bags in the bed of a pickup in the town of Tlapehuala, Guerrero's public safety department said.

Officials said they had not identified the victims, whose remains were found late Thursday in a rural zone, nicknamed Tierra Caliente, or "hot land," that is known for drug-trafficking activity.

Three other people were found dead Friday in separate locations in the Acapulco beach resort, authorities said. Each of the bodies was accompanied by a threatening note signed "the boss of bosses." News media reported at least one other fatality, for a total of 13 bodies discovered in Guerrero in less than 18 hours.

The state's location on the Pacific coast makes it a relay point for smuggling narcotics from South America to the U.S., and coveted turf for warring cartels. Gangs also compete to sell drugs to tourists and residents in the resorts.

Officials said handwritten messages with the decapitated bodies indicated that the killings were carried out by La Familia, a powerful trafficking group based in the neighboring state of Michoacan.

"So that you learn to respect," one note said.

La Familia, a relative newcomer to Mexico's drug underworld, has been locked in an ugly struggle with the Zetas, former allies who serve as the armed wing of the Gulf cartel.

Guerrero is among the most violent states in Mexico, with about 500 drug-related slayings this year. Last weekend, at least 11 people were killed in various locations, including three in Acapulco. Two weeks earlier, four human heads were discovered in ice chests in a Tierra Caliente community known as Coyuca de Catalan, not far from Tlapehuala and near Michoacan.

Also Friday, gunmen in Sinaloa seized two women driving on a highway outside the state capital, Culiacan, authorities said. The women's bodies were found later in a field, their heads wrapped in plastic. Reports in Sinaloa indicated one of the victims was related to Victor Emilio Cazares, whom authorities call one of the top traffickers in the Sinaloa cartel.

ken.ellingwood@latimes.com