Mexican police sent horse-mounted patrols and officers with trained dogs up into the hills around the city of Iguala in an expanded search for 43 college students missing since last month.
The stepped-up hunt was ordered after investigators determined that 28 sets of human remains recovered from a mass grave discovered outside Iguala last weekend were not those of any of the youths who have not been seen since being confronted by police on September 26.
Forensics examinations were focusing on a second set of clandestine graves and a third site where another burial pit was found this week.
The digging that continued yesterday threatened to reveal even greater horrors in the gang-controlled countryside of the southern state of Guerrero.
Each search has turned up more hidden graves, raising the question of how many people have been secretly killed by the area's drug gangs, apart from those kidnapped.
The wooded hillsides that ring Iguala could become a moral swamp for the government, much like the mass graves discovered in northern Mexico in 2010 that revealed a level of almost unheard-of brutality.
"These lamentable acts are a moment that puts to the test the country's institutions," President Enrique Pena Nieto said of the Iguala case.
From the beginning, there were signs that the first mass grave site, found just a few days after the students disappeared, might have contained the bodies of earlier victims of the Guerreros Unidos drug gang.
The gang had ties to the wife of Iguala Mayor Jose Luis Abarca as well as to local police from Iguala and the nearby town of Cocula. Police from the two towns allegedly turned some of the students over to the drug gang.
Isabel Rosales, an activist who works with the families of kidnap victims in Guerrero, said the 28 bodies might be those of victims whose families were too afraid to even report their disappearances.
Given the fact that local authorities are frequently thought to be in cahoots with drug gangs, she said: "There is a lot of fear, terror among the public.
"Out of fear, many people don't even file reports, hoping their loved ones will be released."