After Cassez was detained and held incognito for a day, Mexican police hauled her back to the ranch and forced her to participate in a raid staged for the television cameras, a display that is not unusual in Mexico.
Mexico also has long been plagued by police torture and the fabrication of evidence, and over the years countless prisoners have been convicted on faulty evidence.
Such corruption remains rampant despite a 2008 constitutional amendment to reform the antiquated system from a written, closed trial system to open proceedings with oral arguments. Most of Mexico's 31 states have yet to implement the changes. Even in one that has, Chihuahua, judges were punished for freeing a defendant the public believed to be guilty. They said they were forced to because of improperly gathered evidence.
It's unclear what impact the Cassez ruling will have on defendants' rights and due process in other cases, said John Ackerman of the Institute of Legal Research at Mexico's National Autonomous University.
"The hope is that both the criminal justice reform and this kind of decision would create a demand that crimes be investigated in a more professional manner," Ackerman said. "Just new rules and decisions are not enough. You need institutional transformation and political will and political independence for these investigators, which is something we haven't achieved yet."
Associated Press writers Lori Hinnant, Sarah DiLorenzo and Elaine Ganley in Paris and E. Eduardo Castillo in Mexico City contributed to this report.
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