BATS Global Markets is close to settling a U.S. regulatory probe into Direct Edge's dealings with high-frequency trading firms for $15 million, the biggest fine ever given to an exchange operator, according to two sources familiar with the situation.

The U.S. Securities and Exchange Commission has been investigating how Direct Edge, an exchange that merged with BATS in late January, may have selectively disclosed information about how firms can advantageously place orders on its market prior to the merger.

An announcement is expected as early as Friday, but could be delayed by a week or two, said the people, who did not have permission to speak with the media.

The SEC and BATS declined to comment.

The settlement does not involve an existing and separate investigation into BATS' dealings with high-frequency trading firms, another source familiar with the situation said. That probe is still ongoing.

Previously, the largest penalty ever levied against a stock exchange was $10 million to Nasdaq OMX Group (NDAQ.O) in May 2013 to settle civil charges stemming from mistakes made during Facebook's initial public offering in 2012.

On Tuesday, BATS, the second largest U.S. stock exchange by volume, said it appointed market veteran Chris Concannon as its president, effective Dec. 15.

In July, the company, based near Kansas City, abruptly announced that its president, William O'Brien, had left. Less than six months earlier, BATS merged with Direct Edge, where O'Brien had been chief executive. Joseph Ratterman took on the dual CEO/president role, which he also held from June 2007 through January 2014.

BATS also runs an options exchange and the biggest pan-European stock market.