A U.S. regulator is looking into whether prices linked to the stock market’s widely watched “fear index” have been manipulated, according to people with knowledge of the matter.

The Cboe Volatility Index, known as the VIX, is derived from S&P 500 options prices. The Financial Industry Regulatory Authority is scrutinizing whether traders placed bets on S&P 500 options to influence prices for VIX futures, the people said.

Separately, a letter from a law firm Monday representing an unidentified client urged U.S. regulators to investigate VIX manipulation, claiming it has cost investors hundreds of millions of dollars in losses each month.

If evidence of manipulation is found, it would be a black mark for the VIX, which has soared in popularity over the last decade as a hedging tool for investors. The VIX is designed to track investor anxiety and tends to move in the opposite direction of the benchmark S&P 500 index. Investors purchase VIX futures and options to protect against declines in stocks.

Finra is Wall Street’s self-regulator. It isn’t a government entity but is overseen by the Securities and Exchange Commission. It provides oversight to Wall Street firms such as brokerages and exchanges. Finra works with Cboe Global Markets to help with surveillance of its market.