MORELIA (Mexico) - FEDERAL prosecutors released three of the 10 Mexican mayors arrested last month in an unprecedented sweep of elected officials accused of protecting a drug cartel, authorities said on Monday.

The mayors of the western towns of Coahuayana, Buenavista and Aquila were let go after spending more than a month in jail, the federal Attorney General's Office said in a statement.

The agency did not give a reason for their release or specify whether the investigation of them would continue. Officials could not be reached for further comment.

Seven other mayors arrested in the sweep of 30 officials in the central state of Michoacan remain in prison and have been charged with protecting members of La Familia drug cartel.

Three more officials were arrested on Thursday for suspicion of links to the gang, including Mariano Ortega, mayor of the port city of Lazaro Cardenas, the Attorney General's Office said.

The arrests last month marked an escalation of President Felipe Calderon's efforts to crush ruthless drug gangs and the corrupt officials who protect them.

Mr Calderon said the arrests showed nobody in Mexico is above prosecution: Two of the mayors arrested belonged to his own National Action Party, or PAN. But opponents accused his government of timing the arrests just weeks before the July 5 midterm elections for political gain. -- AP