A concentrated campaign of price manipulation may have accounted for at least half of the increase in the price of Bitcoin and other big cryptocurrencies last year, according to a paper released on Wednesday by an academic with a history of spotting fraud in financial markets.

The paper by John Griffin, a finance professor at the University of Texas, and Amin Shams, a graduate student, is likely to stoke a debate about how much of Bitcoin’s skyrocketing gain last year was caused by the covert actions of a few big players, rather than real demand from investors.