FILE PHOTO -- A sign for the Financial Industry Regulatory Authority (FINRA) is seen outside the offices in New York's financial district July 22, 2015. REUTERS/Brendan McDermid/File Photo

(Reuters) - A Wall Street regulator is probing into whether prices linked to the Cboe Volatility Index .VIX have been manipulated, the Wall Street Journal reported on Tuesday, citing people with knowledge of the matter.

The Financial Industry Regulatory Authority (FINRA) is scrutinizing whether traders placed bets on S&P 500 options in order to influence prices for VIX futures, according to the Journal.

The Cboe Volatility Index is a widely followed gauge of future stock market volatility on Wall Street.

A law firm representing an “anonymous whistleblower” on Monday alleged in a letter to U.S. financial regulators that the VIX is being manipulated, causing billions of dollars in losses a year to unwary investors.

Claims in the letter outline how trading firms have taken advantage of the way the VIX is being calculated in order to manipulate the index.

FINRA was not immediately available for comment.

“Cboe has a dedicated regulatory department that works with FINRA to monitor certain trading activity for our securities markets, including trading activity that could impact the VIX settlement,” Cboe’s Chief Regulatory Officer Greg Hoogasian said in a statement and declined to comment further.