Virtual currency exchanges are popping up at breakneck speed. Tether, which operates USDT, a cryptocurrency that is backed up with the U.S. dollar, announced that almost $31 million of its USDT was stolen from its core treasury wallet “through malicious action by an external attacker.”

In response to the heist, Tether has flagged and is tracking the tokens so no one can exchange them or circulate them through the system. This means that the hacker has the tokens in his/her wallet, but can’t exchange them or profit from them.

Commentators say this is another case that shows how risky investment in virtual currency still is. Although virtual currencies are based on blockchain technology, they can still be stolen and are not backed by the Federal Deposit Insurance Corporation.