Gunmen ambushed and killed 12 police officers who had been sent to investigate the beheadings of 10 people in southern Guerrero state, Mexican authorities said.
Guerrero state police spokesman Arturo Martinez said on Monday that six state and six local officers were killed on Sunday night on a road leading out of the town of Teloloapan. Another 11 officers were wounded.
The attack on the officers occurred as they were travelling in six patrol pickups and searching for the bodies of seven men and three women whose severed heads were dumped outside the town's slaughterhouse earlier on Sunday, Martinez said.
The heads were left with a message threatening the La Familia drug cartel, whose home base is in neighbouring Michoacan state.
Teloloapan is near the area shared by both Guerrero and Michoacan states and known as Tierra Caliente for its steamy weather.
The region is a violent, mountainous zone that has been used by drug traffickers to grow marijuana and opium poppies for years. It has been plagued by drug violence in recent years as drug gangs fight to control the area. Authorities say La Familia has been severely battered in the fighting.
Soldiers have been sent to the area but that hasn't stopped gunmen from killing priests, politicians, police chiefs, or anyone else who gets in the way.
Two years ago, nine police officers were kidnapped in Teloloapan when they were investigating the death of a man in the village of El Revelado. The bodies of eight of the officers were found days later. Six had been dismembered. One was found alive.
More than 47,000 people have died in drug violence nationwide since President Felipe Calderon began a crackdown on drug cartels in December 2006.