In July, US authorities shut down the website of the BTC-e exchange, saying it had “facilitated transactions involving ransomware, computer hacking, identity theft, tax refund fraud schemes, public corruption, and drug trafficking”.
BTC-e, which is no longer operating, could not be reached for comment.
The top three exchanges out of more than 100 - Bitfinex, GDAX and bitFlyer - are home to more than 60 percent of all trading, according to data provider Bitcoinity.
Another issue specific to the market is the risk of hacking and theft. More than 980,000 bitcoins have been stolen from exchanges, Reuters has found, with the Mt. Gox heist accounting for the majority.