By Emily Flitter NEW YORK (Reuters) – U.S. prosecutors in Manhattan are sitting on a multimillion-dollar bitcoin gold mine. Federal authorities hauled in 29,655 units of the digital currency – worth $27 million at current exchange rates – through an official forfeiture by Bitcoin this week. The bitcoins had belonged to Silk Road, an anonymous online black market that authorities say was a conduit for purchases of drugs and computer hacking services – even a place where assassins may have advertised. No one stepped forward to claim these bitcoins, which were found in electronic wallets used to store the digital currency.