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) - The Financial Industry Regulatory Authority (FINRA) has fined a unit of Interactive Brokers Group Inc IBKR.O $5.5 million for violating several naked short selling rules over a period of at least three years.

The unit, Interactive Brokers LLC’s, supervisory system, including its written supervisory procedures, was not reasonably designed to achieve compliance with the federal requirements from July 2012 through June 2015, said FINRA.

FINRA, Wall Street’s self-regulator, also said the company repeatedly ignored “red flags,” including internal audit findings and multiple internal warnings from its staff.

The regulator said Interactive neither admitted nor denied the charges while settling the matter.

Interactive Brokers was not immediately available for comment.