(Reuters) - The U.S. securities regulator on Thursday raised alarm about the safety of bitcoin-themed investments, telling the fund industry they want answers to their concerns before endorsing more than a dozen proposed products based on cryptocurrencies.

FILE PHOTO: A collection of Bitcoin (virtual currency) tokens are displayed in this picture illustration December 8, 2017. REUTERS/Benoit Tessier/Illustration/File Photo

A top division chief at the U.S. Securities and Exchange Commission detailed the agency’s concerns about the wild-trading investment in a letter to two trade groups representing fund managers who unleashed a range of proposals for funds holding bitcoin or related assets.

The SEC’s division of investment management demanded answers to at least 31 detailed questions about how mutual funds or exchange-traded funds based on bitcoin would store, safeguard, and price that asset.

They also asked whether investors can understand the risks and how to address concerns that bitcoin markets could be manipulated. (bit.ly/2DpXsaj)

“There are a number of significant investor protection issues that need to be examined before sponsors begin offering these funds to investors,” said the letter signed by Dalia Blass, the SEC’s director of the division of investment management.

Bitcoin’s 1,500 percent surge last year stoked investor demand for any product with exposure to the red-hot asset. A host of companies are jostling to launch exchange-traded funds which would open up the cryptocurrency to a broad retail market.

The SEC in March denied a request to list an ETF from investors Cameron and Tyler Winklevoss, owners of the Gemini bitcoin exchange.

The Winklevoss fund is seeking to invest in bitcoin directly. Other fund firms staked their hopes on recently launched U.S.-listed bitcoin futures contracts, which promised a more stable base for ETFs than the largely unregulated virtual currency spot market. Many of those proposals were withdrawn last week at the request of the SEC.

Bitcoin was last down a fraction of a percent for the day at $11,228 on the Bitstamp exchange, but it has tumbled more than 40 percent from its peak in December.

Jeremy Senderowicz, a lawyer who represented one proposal for a cryptocurrency product before the SEC, said the SEC statement is a “really big deal” by making public concerns that fund managers would have had to address on a case-by-case basis, behind the scenes.

“It shows that they’re going to have to take some time to consider the industry’s responses before they change their minds on it,” said Senderowicz, a partner at Dechert LLP.

“It gives a template for how to get to a yes.”