Robinhood Markets Inc faces a probe by the U.S. Securities and Exchange Commission over its early failure to fully disclose its practice of selling clients' orders to high-speed traders, the Wall Street Journal reported on Wednesday.

The investigation is at an advanced stage and the company could pay a fine exceeding $10 million, the report added, citing people familiar with the matter.

The fintech startup, which has been credited with helping popularize trading among millennials, is also being investigated by the SEC for its handling of a system outage in March, Bloomberg News reported last month.

A Robinhood spokeswoman said on Wednesday the company strives to cooperate fully with its regulators but does not discuss or comment on its communications with them.

The SEC declined to comment.