MEXICO CITY (AP) — Officially it was a Mexican prison, but inside it hid another reality: a center of operations where the Zetas drug cartel modified vehicles, manufactured uniforms, locked up kidnapped victims and cremated bodies using diesel fuel.

Some details of the case were previously known. But a report released Tuesday by two university professors who specialize in human rights sheds new light on how one of Mexico's bloodiest criminal organizations took complete control of the state prison in 2010 and 2011 in Piedras Negras, just across the border from Texas, without any resistance from...