MEXICO CITY (AP) - Two days of digging outside the city of Monterrey in northern Mexico has produced a gruesome find.
      
Investigators have uncovered 51 corpses in a series of pits at a suspected drug-gang dumping site. They're struggling to identify the remains, but say most have tattoos that could indicate links to drug gangs.
      
The clandestine grave site was discovered Thursday, but the Mexican army did not offer any immediate information on how the site was detected.
      
Monterrey has been the scene of bloody turf battles between a pair of rival drug cartels.
      
Nearly 25,000 people have been killed in Mexico since the government launched an offensive against drug cartels in late 2006.