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.

“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.