A growing number of virtual currency investors are worried that the prices of Bitcoin and other digital tokens have been artificially propped up by a widely used exchange called Bitfinex, which has a checkered history of hacks and opaque business practices.

In December, Bitfinex was subpoenaed by the Commodity Futures Trading Commission, a United States regulatory agency. The news, first reported by Bloomberg on Tuesday and confirmed by a source familiar with the subpoena but not allowed to publicly discuss an ongoing investigation, led to a sell-off in most virtual currencies.

The people behind Bitfinex issue a virtual currency called Tether. Unlike most digitals tokens, every Tether is supposed to be backed by traditional money — the United States dollar. New Tether tokens are issued when investors give them dollars. One dollar is worth one token.

Because of the credibility that comes with that tie to the dollar, Tether are often used to buy other virtual currencies like Bitcoin.

In recent months, however, many investors have been raising alarm bells about Tether. Hundreds of millions of dollars worth of new Tether were created; almost always when the prices of other virtual currencies were heading down. The Tether were used on the Bitfinex exchange to make big purchases of Bitcoin and other tokens, helping push their prices back up, according to multiple analyses of data from Bitfinex.

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"This became more and more concerning, because every time the markets went down, you have seen the same thing happen," said Joey Krug, the co-chief investment officer at Pantera Capital, which runs several virtual currency hedge funds. "It could mean that a lot of the rally over December and January might not have been real."

Long before news of the subpoena, Bitfinex, which is believed to host more trading than any other Bitcoin exchange in the world, had gained a reputation for a lack of transparency and a confusing structure, with European executives, offices in Asia and registration in the Caribbean.

It is not yet clear what information the regulators are seeking. Technically, the Tether tokens are issued by a separate company (called Tether) that is owned and operated by the same people who run Bitfinex. The C.F.T.C. subpoenaed that company at the same time that it subpoenaed Bitfinex, according to a person familiar with the matter.

Bitfinex has not commented on the subpoena or recent reports about Tether, and company officials did not respond to repeated requests for comment. In the past, the exchange's executives and spokesman have said that its customers are simply using Tether to buy virtual currencies as they might otherwise use United States dollars.

Bitfinex had contracted with an American firm, Friedman, to audit its records and prove that its operation of Tether is above board. But last week, Bitfinex said it was cutting ties with Friedman, after waiting months for it to finish the audit. That news generated more suspicion.

"It's a signal to the market of what those who have scrutinized the situation already believe: There is a problem here," said Jill Carlson, a former trader at Goldman Sachs who now consults with a variety of virtual currency companies. "The dissolution of a relationship between an auditor and a company is very rarely a good sign that the company is behaving in accordance with market best practices."

The concern about Bitfinex is one of several issues that have helped depress the value of virtual currencies over the last month, after a roaring, yearlong rally. Regulators in several countries, like South Korea and the United States, have expressed concerns about manipulation and fraud, and hinted at a crackdown.

In Japan, a large virtual currency exchange, Coincheck, was hacked in late January and lost nearly $500 million worth of a virtual currency known as NEM, raising questions about the relatively untested security practices of virtual currency exchanges.

Still, Japan is one of the few places where virtual currency exchanges are overseen by regulators. Many of the largest virtual currency exchanges, including Bitfinex, operate with essentially no regulatory oversight.

Bitfinex was hacked in 2015 and again in 2016, and Tether was hacked at the end of last year — with the combined losses totaling more than $100 million.

The company shared few details about these hacks. But after the biggest theft, in 2016, it was cut off by Wells Fargo and its banks in Taiwan.

The banking problems have made it hard for customers to get money in and out of Bitfinex, but traders have continued to use the exchange, in part because of Tether.

Tether offered a preliminary report last year from Friedman, the accounting firm, suggesting that it had bank accounts with dollars corresponding to all the Tether that had been issued. But the report was far from conclusive and Tether has never produced a real audit, leading to suspicions that Bitfinex may be printing virtual money backed by nothing.

The Tether currency has been valuable to traders because it allows them to hold a stable token, tied to the value of the dollar, and move it quickly between virtual currency exchanges.

But there is a downside. Because the identity of Tether holders is not always clear, the movement of the virtual token between exchanges — and across national borders — has raised concerns among lawyers about money laundering. The spread of Tether beyond Bitfinex has also spread the risk of its failure to other exchanges.

Market analysts have grown particularly concerned with the rapid pace at which new Tether have been issued and their timing. In a single week in mid-January, $450 million new Tether were created, bringing the total amount of Tether to more than $2 billion.

Several anonymous reports circulating among traders, including one posted to a website last week, have pointed to data from Bitfinex itself, showing that the price of Bitcoin has frequently gone up soon after new Tether were created, generally as a result of big trades on Bitfinex.

"This absolutely reeks of price manipulation," a security researcher and market analyst, Tony Arcieri, wrote on his blog in mid-January.

Many investors and traders have pushed back against that criticism, noting that smart traders simply may be looking to buy when the markets are down.

"My personal biases make me inclined to believe that Bitfinex is not using Tether to manipulate the price of Bitcoin," said Jeremy Gardner, a managing partner at the investment firm Ausum Ventures. "If they have attempted to do so, which would be deeply concerning, it's hard to imagine that such fraudulent issuance at its current volume could single-handedly buoy the price of Bitcoin."

But Mr. Krug, at Pantera Capital, said that if Tether were really being used by investors, they would probably also want to buy new Tether when the markets were going up, which has not been the case. Also, they would not always want it in exact increments of $100 million, as has been the case.

"After you see this enough times, you just start to wonder what's really going on here," Mr. Krug said.

The Bitcoin community is sensitive to the possibility of price manipulation because a team of academics published an article in early January suggesting that the price of Bitcoin was artificially inflated in 2013 by a single player operating on the largest exchange at the time, Mt. Gox.

One of the author's of that paper, Tyler Moore, said it could be hard to tell if similar price manipulation were going on today, though he noted that a lack of transparency made it hard for anyone to be certain of anything.

"Greater assurances are needed that the trades taking place are in fact legitimate and reflect buying and selling by independent actors," said Mr. Moore, an assistant professor at the University of Tulsa. "Unless and until such oversight is implemented, we cannot trust the exchange rate to reflect only legitimate sources of supply and demand."