The Securities and Exchange Commission rejected an application on Friday that would have opened the door for the first exchange-traded fund for the virtual currency bitcoin.

The SEC denied a request from Cameron and Tyler Winklevoss, the twins famous for suing Facebook founder Mark Zuckerberg, related to a proposed ETF that would track the price of bitcoin and could be bought and sold as easily as stocks. The commission said it rejected the application because the bitcoin exchanges that are now used to buy and sell the virtual currency are not regulated. The SEC said in its analysis that the exchanges are susceptible to “fraudulent or manipulative acts and practices.”

Tyler Winklevoss said in an emailed statement that they would keep working on the ETF. “We remain optimistic and committed to bringing COIN to market, and look forward to continuing to work with the SEC staff. We began this journey almost four years ago, and are determined to see it through. We agree with the SEC that regulation and oversight are important to the health of any marketplace and the safety of all investors.” The Winklevosses had requested for the ETF to trade on the Bats BZX Exchange, a stock exchange.

The virtual currency, which was created in 2010, has had a checkered past in part because of the anonymity it allows users. The currency gained traction as a fast way to send money internationally, but has also been criticized as being vulnerable to hacking or technical crashes.

A bitcoin ETF would allow people to invest in the digital currency in the same way that they can buy funds that track tangible currencies such as the dollar, the yen or the euro, said Todd Rosenbluth, director of ETF and mutual fund research at CFRA. But unlike the more conventional currencies, which are influenced by trends in the economy, decisions by central banks and other metrics, it is more difficult to identify the forces influencing the price of bitcoin, Rosenbluth said.

In its decision, the SEC said that commentators expressed concerns that bitcoin exchanges are subject to price “volatility and instability.” Bitcoin prices have fluctuated as investors have swung from being optimistic about the potentially positive effects of an ETF to being concerned about reports of security breaches or potential investigations.

The price of bitcoin swung widely on Friday as investors worried about the fate of the ETF. The price plunged as low as $1,066 following the decision after surging to a record high above $1,300 earlier in the day, according to the Bitcoin Price Index from CoinDesk, a website that tracks digital currencies.

The rejection may put a damper on other efforts to make the virtual currency widely available to investors through ETFs. At least two other requests for bitcoin ETFs are still pending with the SEC from Grayscale Investments and SolidX.