Bitcoin tumbled more than 10 percent over the weekend to its lowest in two months, after a relatively small South Korean exchange said it was hacked.
Over the weekend, crypto exchange Coinrail tweeted that it was hacked, and noted that lesser-known cryptocurrencies such as Pundi X were among those affected, according to Google Translate. The Pundi X-bitcoin pair is the most-traded on Coinrail, CoinMarketCap data showed.
However, Coinrail's public statements did not mention bitcoin, according to Google Translate.
Nevertheless, Bitcoin hit a low of $6,647.33 on Sunday, its lowest since April 9, according to CoinDesk's bitcoin price index, and coincided with a broader crypto sell-off. The digital currency traded off those lows Monday afternoon, near $6,700.
The largest cryptocurrency by market capitalization remains about 50 percent lower for the year so far.
Bitcoin three-month performance
Other major cryptocurrencies also fell. Ethereum dropped more than 11 percent to near $533, according to CoinDesk.
The declines followed a report on Friday from The Wall Street Journal, citing sources, that U.S. regulators are investigating potential price manipulation at four major cryptocurrency exchanges: Bitstamp, Coinbase, itBit and Kraken.
"We support all market participants, exchanges, regulators in having a dialogue to enhance transparency and find common ground," Chad Cascarilla, CEO and co-founder of Paxos, which owns cryptocurrency exchange itBit, said in a statement. The three other cryptocurrency companies did not immediately respond to a CNBC request for comment on Sunday evening.
CME said exchanges contributing to its Bitcoin Reference Rate — which include the four above — are required to share information and cooperate with regulators. The Commodity Futures Trading Commission would not confirm or deny any investigative activity.