Federal officials remain vulnerable in the state of Michoacan
Ten Mexican police officers have been detained in connection with the torture and murder of 12 federal agents during a major escalation in the drug war.
The arrests come as more than 5,000 troops and federal police are deployed in the western state of Michoacan.
The troop surge, one of the biggest in the anti-drugs campaign, comes after a local drug gang launched co-ordinated attacks in 10 cities last week.
The state governor has protested against the "military occupation".
The federal authorities say they are investigating links between the municipal police and drug traffickers in the murder of the agents, whose bodies were found bound and gagged and shot through the head next to a major highway.
In a statement, prosecutors said the detentions would enable them to strengthen evidence that the officers "undertook criminal acts" in support of the Michoacan drugs gang and to "determine their responsibility for the murder of federal agents".
Earlier this year 10 mayors in the state were arrested by the federal authorities on suspicion they were working with the drug gangs.
Troops with automatic weapons and ski masks to shield their identity have set up roadblocks across Michoacan, President Felipe Calderon's home state, in a major show of force.
Nineteen police were arrested in one small town, 10 of whom are still being held in custody while alleged links with drug gangs are investigated.
The federal government believes that local police and officials have long been in the pay of the drug gangs.
The Michoacan gang, known as the "Family", announced itself as a terrifying new force three years ago when its hitmen tossed the severed heads of five victims onto a dancefloor in a city nightclub.
Despite the roadblocks, analysts say federal agents remain highly vulnerable in a region where drug gangs can easily get intelligence about their movements.