It was a brazen attack. Some 60 gunmen linked to the brutal Zetas cartel descended on a quiet cluster of towns just south of the Mexican border in the spring of 2011 and launched a door-to-door extermination campaign that went on for weeks, leaving an untold number of people dead or missing. Yet in the five years since the slaughter in the northern Mexican state of Coahuila, the Mexican government has failed to fully investigate, much less address the needs of the victims and their families, according to a preliminary report released today by a panel of scholars and human rights investigators. “It’s horrifying because it was all so blatant,” said Mariclaire Acosta, a veteran human rights investigator who advised the panel. “This wasn’t a hidden crime.