At least one victim identified Cassez as one of the kidnappers, though only by hearing her voice, not by seeing her.
"In this country we can no longer ignore police obtaining evidence by tampering with it, by using torture, by staging raids," said Luis Gonzalez Placencia, president of Mexico City's Human Rights Commission. "We will never know whether Florence is guilty or innocent, but we know for certain there are specific people who violated due process."
It was not immediately clear how the ruling might affect the case against Cassez's ex-boyfriend Israel Vallarta, who was arrested for allegedly leading the gang and is being tried separately.
Anti-crime activist Isabel Miranda de Wallace, who led a successful decade-long fight to bring her son's kidnappers to justice even though his body was never found, lashed out at the court's ruling.
"Today, they opened the door to impunity, today a lot of people are going to go free," Miranda de Wallace told local media. "We already live without public safety, now it's going to be worse."
Ezequiel Elizalde, a kidnap victim who testified against Cassez, told local media the Mexican justice system was discredited by the ruling, and that citizens should no longer depend on it. "Get a weapon, arm yourself, and don't pay any attention to the government."
Cassez was originally sentenced in 2008 to 96 years in prison for four kidnappings. The sentence was reduced to 70 years a year later when she was acquitted of one of the charges.
The case caused diplomatic tensions between France and Mexico. In 2011, the Mexican government said it would not participate in France's yearlong festival celebrating Mexican culture, after then-French President Nicolas Sarkozy said the festival should be used to draw attention to the Cassez case.
The case showed Mexico's legal system to be plagued by irregularities and a slow process.
Mexico in 2008 implemented a judicial reform that called for open trials and reinforced the principle of innocence until proven guilty. The old system, still in place in most of the country, was blamed for fostering corruption and confessions extracted by torture.
The country's Supreme Court has grown more independent in recent years, and the public has become more willing to acknowledge the shortcomings of the justice system. A widely viewed documentary film, "Presumed Guilty," detailed the story of a man arrested off the street and held for several years for a murder he didn't commit.
Associated Press writers Olga R. Rodriguez in Mexico and Sarah DiLorenzo in Paris contributed to this report.
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