Gunmen in western Mexico have killed 12 policemen investigating the beheadings of 10 other people.
The officers were attacked as they searched for bodies after severed heads were found near the town of Teloloapan in Guerrero state on Sunday.
Messages threatening the La Familia drug cartel were found with the heads.
Correspondents say La Familia and the Knights Templar, a rival cartel, are fighting for control of the drugs trade in Guerrero state.
Few details gave been release about the ambush, which occurred late on Sunday. At least nine agents were injured and are being treated in hospital.
Some 4,000 Mexican police and army are involved in operations against drug traffickers in the area, where major tourist destinations like Acapulco and Zihuatanejo are located.
However, the problem is still rampant - 50,000 people have been killed in drug-related violence in Mexico since 2006.
Pope Benedict is due to arrive in Mexico on Friday and is expected to comment on drug-related violence in the country.
The BBC's Will Grant in Mexico says people are hoping for a respite from the violence during the Pope's visit - but they may not get one.