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MEXICO CITY -- A letter allegedly signed by organized-crime leaders offers to dissolve a major Mexican drug cartel if the government promises to protect citizens in the western state where it is based, authorities said Wednesday.

Prosecutors said that they couldn't immediately verify the letter's authenticity -- or the offer's sincerity -- but stressed the federal government does not negotiate with drug cartels.

The one-page letter allegedly signed by La Familia drug cartel was dropped in the streets of some mountain towns in the western state of Michoacan on Tuesday, according to the Michoacan bureau of the federal Attorney General's Office. It also showed up as a banner above an overpass and was sent as an e-mail to reporters.

The missive claims La Familia wants to protect Michoacan and its residents and says the group will disband if federal police promise to act honestly and fight to the death to defend the state.

"We have decided to retreat and return to our daily productive activities if the federal and local authorities ... promise to take control of the state with force and decision," read the letter, dated November 2010. "If the government accepts this public commitment and lives up to it, La Familia Michoacana will dissolve."

Federal officials, however, say the cartel itself has victimized Michoacan with kidnappings, extortion, hundreds of murders, decapitations and drug trafficking. Last year, they say, the gang unleashed a spasm of violence in which at least 18 police officers were killed. Last week, in response to the arrest of two members, the gang set ablaze trucks to block entries to the state capital and sprayed a shopping mall with automatic-weapons fire. A civilian driver was injured when assailants stole his bus to use in the blockades, the state attorney general's office said.

The letter allegedly written by La Familia says the gang's decision to possibly dissolve was motivated by alleged abuses against civilians by authorities conducting warrantless searches and arrests to combat the cartel.

An employee of the Attorney General's Office in Michoacan said authorities were investigating the letter's origin but could not immediately confirm its authenticity. The employee spoke on condition of anonymity because he was not authorized to be quoted by name.

Ricardo Najera, spokesman for the federal Attorney General's Office, said that "regardless of whether the message is authentic or not, the federal government does not make deals or negotiate with drug cartels."

La Familia appears to have deep local roots and an extensive network of civilian collaborators and sympathizers.

It also has distinguished itself by occasionally making public pronouncements, and has issued a set of rules for cartel members that proclaim family values and prohibit consuming -- but not trafficking -- hard drugs.

The cartel also has sought to convince the public that it is defending Michoacan -- where President Felipe Calderon was born -- against other drug groups.

The letter said La Familia was formed in 2005 "by men and women from Michoacan ready to give their lives to defend their state ... against external gangs that, through terror and violence, have attempted to take over not only our state, but the whole country."