Federal regulators have again rejected efforts by entrepreneurs Cameron and Tyler Winklevoss to launch a bitcoin-focused exchange-traded fund (ETF), which could have allowed supporters of bitcoin to invest in the virtual currency without buying and holding the digital assets directly.

In a 92-page order Thursday, the Securities and Exchange Commission said the company behind the listing effort, Bats BZX Exchange, failed to prove that the bitcoin market would be sufficiently resistant to fraud and manipulation. Although Bats argued that the bitcoin market “generally is less susceptible to manipulation” because bitcoin trading happens around the world continuously, the SEC said it was not persuaded.

“The record before the Commission does not support such a conclusion,” the order says.

The decision is a blow to the Winklevoss brothers, who have spent years seeking to bring cryptocurrency investing into the mainstream. Despite a surge in bitcoin’s popularity last year — prompting coin prices to hit more than $19,000 — the total market cap of all digital currencies still pales in comparison to the $30 trillion market cap of the U.S. stock market.

Launching a bitcoin ETF could make it easier for investors to gain exposure to the cryptocurrency, analysts have said. But the SEC’s latest move is the second time since March that regulators have dismissed the idea. Prices of bitcoin fell roughly 2 percent late Thursday afternoon, according to CoinMarketCap.com.

The Winklevosses could not be reached for comment.

The SEC’s decision may be appealed to a federal court. Still, the agency did not close the door completely to bitcoin ETFs. Should circumstances arise that make it easier to detect fraud and manipulation, regulators said, the SEC would be open to reviewing the rules. And, it added, its decision Thursday did not reflect a judgment on the technology behind bitcoin.

“The Commission emphasizes that its disapproval does not rest on an evaluation of whether bitcoin, or blockchain technology more generally, has utility or value as an innovation or an investment,” the order said.

One member of the four-person commission voted to bless the Winklevoss initiative. In a dissent released alongside the SEC’s decision, Commissioner Hester Peirce said that by not bringing bitcoin investing into the mainstream, the agency would be missing an opportunity to institutionalize it and make the markets safer.

“It precludes investors from accessing bitcoin through an exchange-listed avenue that offers predictability, transparency, and ease of entry and exit,” said Peirce in her dissent. She added that the SEC’s analysis focused unnecessarily on the bitcoin market when it should have narrowly addressed BZX’s ability to regulate itself.