ACAPULCO, Mexico (AP) — Mexican authorities are investigating the possible emergence of a new drug gang that appeared to take credit for six killings through a message left with the bodies Monday, officials said.

The six men were found inside a car in the southwestern city of Chilpancingo, Guerrero state police said in a statement. Next to them lay a message reading: "This will happen to all rapists, extortionists and kidnappers. Attentively, the New Cartel of the Sierra."

Authorities are investigating the authenticity of the gang, said an official with the state prosecutors office, who spoke on condition of anonymity because he was not authorized to speak publicly about the case. The official said authorities had no previous knowledge of such an organized crime group.

The car was reported stolen hours before the bodies were found, according to the police statement. The owner told police that armed men intercepted him on a highway and forced him out of the car.

One body was found stuffed inside a black bag and the rest were tied up.

At least seven major drug trafficking cartels operate in Mexico, but there are many smaller gangs throughout the country, often affiliated with one of the bigger groups.

The cartels have increasingly splintered since President Felipe Calderon launched an intensified crackdown after taking office in late 2006, deploying thousands of troops and federal police across Mexico.

Mexican authorities have blamed the infighting for a surge of gang violence that has killed nearly 25,000 people in less than four years.

Most recently, a fight for control of the Beltran Leyva cartel has increased violence in central and southwestern Mexico, including Guerrero state, which is home to the resort city of Acapulco.

The Beltran Leyva cartel splintered after its leader, Arturo Beltran Leyva, was killed in a gunbattle with Mexican marines in December. That split occurred only a year after the Beltran Leyva gang broke with the Sinaloa cartel, which remains one of the world's most powerful drug trafficking organizations.

Violence has also surged this year along Mexico's northeastern border with the U.S. since the Gulf cartel split with its former gang of enforcers, the Zetas.

In that region, the bodies of four men were found dumped in a plaza Monday in Nuevo Laredo, a city across the border from Laredo, Texas, the Tamaulipas state police said in statement. The bodies had signs of torture and were found with the remains of a dog and a cat and several threatening messages.

In the city of Cuernavaca, south of Mexico City, state investigators found the burned bodies of three men near a major highway. The victims had been bound before being burned. Painted on a nearby wall were the letters "CPS," an apparent reference to the Southern Pacific drug cartel.