Federal police in Acapulco said they caught a young woman and a young man as they were getting out of a car near a shopping mall with an ice chest that contained a decapitated head and other human remains. Police had followed them because the car matched the description in a kidnapping report.
Inside the vehicle, police said, they found another head inside another ice chest.
Police officers discovered the bound bodies of the victims in a car near Acapulco two hours after the arrest.
Federal police said 19-year-old Damaris Gomez leads a group if killers working for "the street sweeper" gang. The 21-year-old man riding with her is an alleged hit man, police said.
The local gang has been fighting the Independent Cartel of Acapulco for control of the coast city since the 2010 arrest of Edgar Valdez Villarreal, known as "La Barbie." He is accused of being a drug capo for the Beltran Leyva cartel
Mexican President Felipe Calderon has deployed federal police officers to the port city to curb drug violence.
Meanwhile, the Mexican army says it has found an auto shop used to bulletproof vehicles for drug gangs.
Ten people were arrested and soldiers confiscated 10 cars or SUVs that were being bulletproofed, as well a six other vehicles in a warehouse in the northern state of Sinaloa, the army said in a late-Monday statement.
Authorities did not say which cartel managed the shop raided on Sunday, but it was in the home state of Mexico's most-wanted drug lord Joaquin Guzman, who is known as "El Chapo."
In border states such as Tamaulipas, soldiers have raided similar bulletproofing shops and found freight trucks completely covered in steel.