The cryptocurrency world, with its volatility, is all about FUD—fear, uncertainty, doubt. And nothing is generating more FUD right now than an unusual currency called tether.

Unlike bitcoin and its many siblings, tether is what is called a stablecoin, an entity designed to not fluctuate in value. With most cryptocurrencies prone to wild swings, tether offers people who dabble in the market the option of buying a currency that its backers say is pegged to the US dollar. Trading bitcoin for dollars at a bank can be cumbersome and costly; by comparison, acquiring tether is simple, cheap and fast.

But in recent weeks a chorus of skeptics has called into question nearly everything about tether. The root of the controversy is whether the company behind it, also called Tether, is telling the truth when it claims that every unit in circulation is matched by a US dollar it holds in reserve. If the company has a dollar for every tether, that means in theory any holder can sell tethers back to the company for an equal number of dollars at any time. This belief keeps the value of a tether pegged to a dollar.

Critics on Twitter, Reddit, in blog posts, and at a recent bitcoin conference have been demanding that the company prove its reserves through external audits. Not only has Tether failed to do so, last week it confirmed rumors that it had severed ties with Friedman LLP, the accounting firm on tap to perform those audits. On Tuesday, Bloomberg reported that the US Commodity Futures Trading Commission had sent subpoenas to Tether. A Tether spokesperson said, "We routinely receive legal process from law enforcement agents and regulators conducting investigations. It is our policy not to comment on any such requests." The spokesperson declined other comment.

If tethers are not backed by a matching number of dollars, then Tether can print an arbitrary amount of money. (Other cryptocurrencies, by contrast, create new tokens according to strictly prescribed, predictable rules.) Other problems ensue, including suspicions that Tether is timing the release of new tethers to coincide with drops in the price of bitcoin and then using those tethers to scoop up bitcoins. Some observers fear that these purchases are artificially inflating the price of bitcoin. “It’s possible that a nontrivial rise in the price of bitcoin and other cryptocurrencies has come from this asset being printed possibly out of thin air, and that is very concerning,” says Jill Carlson, a former Wall Street trader who now invests in and consults for cryptocurrency startups.

If traders lose faith in tether, they could end up triggering the crypto version of a bank run. Tether helps stabilize cryptocurrency exchanges in various ways, so its collapse could also cause some exchanges to topple, wiping out billions of dollars of investments overnight and potentially undoing much of the public’s growing interest in new technologies like bitcoin.

“It’s possible that a nontrivial rise in the price of bitcoin and other cryptocurrencies has come from this asset being printed possibly out of thin air, and that is very concerning,” says Jill Carlson.

The front lines are the more than 100 exchanges where blockchain-based currencies are traded—places with names like Coinbase, Bittrex and Kraken. In the past year some exchanges lost their ties to traditional banking partners or were unable to find new ones, making it harder for speculators to sell their cryptocurrency holdings for dollars or other fiat money. Tether grew popular in this climate because it offered traders a way to escape the volatility. They could buy tethers with some confidence that the currency would not suddenly plummet in value.

Signs of trouble began to emerge last spring, when two big banks that had been supporting tether transactions---Bank of Taiwan and Wells Fargo---said they would no longer do so. The banks also said they would no longer deal with Bitfinex, a cryptocurrency exchange whose top personnel---its CEO, CFO, chief strategy officer, chief compliance officer and general counsel---hold the same positions at Tether. Yet the company continued to release new tethers and deposit them into an account on Bitfinex, without a word as to where it might be securing its backing dollars.