At least 13 people were gunned down in Mexico, where six human heads, including those of two missing police officers, were also found, officials said.
Seven people were shot to death in Tijuana, a border city in Baja California state, while six others were killed in the southern state of Guerrero.
Five of the victims in Tijuana, located near San Diego, California, were killed Sunday night and appeared to be drug dealers, the Baja California state Attorney General’s Office said.
The other two people slain in the northern border city died early Monday, the AG’s office said.
The five street dealers, one of whom was a woman, were killed execution-style inside a two-bedroom house in a poor section of the border city, prosecutors said.
One of the victims was pronounced dead on arrival at a hospital, the AG’s office said.
Unofficial reports said the five were killed as a result of a drug deal that went bad.
The other two people killed in Tijuana were a 20-year-old man and a 32-year-old man who apparently owned several taxis, prosecutors said.
Police in the southern state of Guerrero said six people were killed, with the bodies found at different places along a rough dirt track.
The first three bodies found, which belonged to two women and a man, were left in Tlanipatlan, a community near the city of Apaxtla, two other bodies were discovered inside a vehicle a kilometer away and the sixth body was found inside another automobile several kilometers away.
Officials received reports that members of organized crime groups engaged in a shootout in the area, the Guerrero Public Safety Secretariat said.
Six human heads, meanwhile, were found in the cities of Lerdo and Gomez Palacio, located in the northern state of Durango.
Two of the heads belonged to Gomez Palacio municipal police officers Jose Refugio Olivas Cereceros and Sebastian Moreno Castro, both 32, who were reported missing last week.
The officers’ bodies were found Sunday near Gomez Palacio, the Durango state Attorney General’s Office said.
The patrol car assigned to Olivas Cereceros and Moreno Castro was found Thursday in a field near the road that links Gomez Palacio to Tlahualilo.
The other four victims have not been identified, prosecutors said.
A cooler containing a man’s head was found Sunday on a street in Lerdo.
Three decapitated bodies were found inside a car abandoned under a bridge in Gomez Palacio. Two human heads were on the windshield and the other was inside the vehicle.
Police found two bodies in the trunk and the other on the back seat.
Mexican drug traffickers often decapitate their victims to send a message to rivals or those thinking about betraying them.