Bottom line: Remember when Bitcoin hit its highest price of over $19,000 last year? Other cryptocurrencies, including ethereum and many altcoins, also skyrocketed, but researchers say this was the result of market manipulation.

A paper published by University of Texas professor John Griffin, who is famed for identifying fraud in financial markets, and graduate student Amin Shams explains how digital currency Tether may have been used to inflate the cryptos' prices.

Tether is a token that's directly linked to the US dollar. Each one is worth $1 and is (supposedly) backed by physical dollar holdings, meaning users can convert their real-world cash into Tether and easily spend it on digital currencies.

The report suggests that popular exchange Bitfinex, which created and sold Tether, may have used it to buy up Bitcoin and increase demand, thereby boosting its price. The paper says the more Tether that was released into the market, the higher cryptocurrency prices would rise, "similar to the inflationary effect of printing additional money."

"By mapping the blockchains of bitcoin and Tether, we are able to establish that entities associated with the Bitfinex exchange used Tether to purchase bitcoin when prices were falling," the report explains. "Such price supporting activities were successful, as bitcoin prices rose following the periods of intervention. These effects are present only after negative returns and periods following the printing [sic] of Tether."

"There were obviously tremendous price increases last year and this paper indicates that manipulation played a large part in those price increases," Griffin told The New York Times.

Bitfinex stopped issuing Tether earlier this year. It's current market capitalization is $2.5 billion.

Jan Ludovicus van der Velde, the CEO of both Bitfinex and the company Tether, has refuted the findings. "Bitfinex nor Tether is, or has ever, engaged in any sort of market or price manipulation," he told Business Insider in a statement. "Tether issuances cannot be used to prop up the price of Bitcoin or any other coin/token on Bitfinex."

Bitcoin and other virtual currencies fell yesterday. From its near-$20,000 high in December, the crypto dropped to $6,107 at one point yesterday---its lowest since early February.

Bitcoin's price over the last 12 months