Garantex, Russia's largest sanctioned crypto exchange, has halted operations after Tether froze over 2.5 billion rubles ($27 million) in USDT.Tether's action is in response to Garantex's violation of anti-money laundering laws and processing of illegal transactions.The European Union and the U.S. have previously sanctioned Garantex for these violations.Officials are investigating over $20 billion in USDT transactions processed through Garantex, with Tether's involvement in question.