Tan Seet Eng, also known by the nickname Dan Tan, was jailed in October 2013 after Italian prosecutors accused him of coordinating a global crime syndicate that made millions of dollars betting on rigged Italian matches and other games across the world.
Tan was held under a Singaporean law that allows for indefinite detention without trial if it's in the interest of public safety, but Chief Justice Sundaresh Menon of the Singapore Court of Appeal said Wednesday there was no evidence to suggest he was a safety risk.
"The matches fixed, whether or not successfully, all took place beyond our shores," he added. "There is nothing in the grounds to indicate (he) was working with overseas criminal syndicates or to suggest that such activities are likely to take root in Singapore, by reason of anything (he) has done or threatens to do."
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One of Tan's lawyers, Hamidul Haq, said Wednesday that he had long argued Tan was being held on unlawful grounds.
"It has been proven true today through the court judgment," he said. "My client is very relieved."
Introduced in 1955, the Singaporean detention law has been used against suspected drug traffickers, illegal money-lenders and criminal gang members, especially in cases involving insufficient evidence for prosecution.
Tan was arrested along with 13 others in September 2013 in a move that was hailed by Interpol as a major breakthrough in the battle against corruption in soccer.
Tan was suspected of being the mastermind behind fixed matches in Italy's Serie A and Serie B in 2011. He is also being tried in absentia by a Hungarian court for allegedly manipulating 32 games in Hungary, Italy and Finland.