A letter purportedly signed by La Familia drug cartel announcing a one-month truce circulated in Mexico's western state of Michoacan.
In the one-page message, distributed by email and in some cities door to door, the gang claims it will halt all crime activity during January to demonstrate that the cartel "is not responsible for the criminal acts federal authorities are reporting to the media".
Prosecutors have not verified the letter's authenticity, according to an employee of the Michoacan bureau of the federal Attorney General's Office.
The government said La Familia has been weakened by a recent string of arrests and deaths of top leaders.
In another letter that circulated in November, La Familia purportedly offered to disband.
Last month, gunmen torched vehicles across Michoacan and used them as barricades to block all entrances into the state capital of Morelia after federal police killed alleged La Familia leader Nazario Moreno Gonzalez.
La Familia has occasionally made public pronouncements seeking to convince the public that it is defending Michoacan against other drug gangs.
Federal officials, however, say the cartel has terrorised the state with kidnappings, extortion, hundreds of murders, decapitations and drug trafficking.
More than 30,000 people have died in drug-related violence nationwide since President Felipe Calderon launched a crackdown on cartels after taking office in December 2006, first deploying hundreds of soldiers and federal police to his home state of Michoacan.
On Sunday, Mr Calderon said in a New Year's broadcast that his administration will continue to fight organised crime. "We all know it is necessary to rid Mexico of crime, impunity and corruption, which had been rooted in our society and our institutions," he said. "I can assure you we are on the right path and we will defeat the criminals, to ultimately build a Mexico of peace."