By November of that year, according to people close to the attorney general’s investigation, Bitfinex determined that it had permanently lost access to the $850 million. To hide the missing funds, Bitfinex and Tether engaged in a series of maneuvers that drained Tether’s reserves, the people said. A gap of that size would represent a major portion of Tether’s reserves. Tether currently claims on its website that the coins it issues are backed by reserves that include currency, cash equivalents and other assets and receivables. The language was altered in March; it previously claimed the reserves were 100% in currency.

https://www.wsj.com/articles/bitfinex-used-tether-reserves-to-mask-missing-850-million-probe-finds-11556227031?mod=searchresults&page=1&pos=1