Members of the doomsday Aum Supreme Truth cult released sarin gas in the Tokyo subway system in 1995, killing 13 people and sickening thousands of commuters in a crime that deeply scarred Japan's sense of security.
Former Aum member Naoko Kikuchi was not prosecuted for the subway assault with sarin gas, first developed by the Nazis, but was instead convicted for a separate attack on a government building.
Kikuchi, 43, appealed the verdict and today the Tokyo High Court ruled that "the defendant is not guilty," a court spokeswoman told AFP, declining to give further details.
The court found that Kikuchi was not aware of her role in the bombing, according to Jiji Press and other media.
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Her lawyers had argued that she was not informed of the details of the plan to kill people with the bomb, Jiji said.
Kikuchi, who had been one of only two remaining members of the Aum cult at large, was arrested in June 2012. Less than two weeks later, the final fugitive was also arrested.
The cult was led by Shoko Asahara, a partially blind guru who preached a blend of Buddhist and Hindu dogma mixed with apocalyptic messages.
Asahara was arrested at a commune near Mount Fuji two months after the subway attack and sentenced to hang in 2004, having been convicted of crimes resulting in multiple deaths.