An armed gang disguised as police has broken into a jail in northern Mexico and freed more than 50 prisoners. A convoy of at least 15 vehicles arrived at the prison with gang members all apparently disguised as federal police officers. Once inside, the prison authorities were persuaded that an inspection was under way and guards were overpowered. A manhunt is underway for the fugitives - believed to be linked to the powerful drug smuggling Gulf cartel. Army troops as well as local and federal police have begun a search and set up roadblocks near the prison facility in Zacatecas and neighbouring states. President Felipe Calderon has declared war on the illegal narcotic organization in Mexico, deploying 40,000 troops to fight the cartels.