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MOSCOW (Reuters) - A Russian national arrested in Greece on Wednesday on suspicion of laundering criminal funds by switching them into bitcoins is a key person behind the BTC-e crypto-currency exchange, two sources close to the exchange told Reuters.BTC-e, one of the oldest trading platforms for digital currencies, did not immediately respond to a request for comment.Police sources identified the man as Alexander Vinnik, 38, who was arrested in northern Greece on a U.S. warrant and is accused of laundering at least $4 billion.