The U.S. Securities and Exchange Commission’s widely hailed whistleblower program has paid millions in recent years to former SEC lawyers who have come to dominate the market for representing tipsters seeking payouts through the program, a new study found.

An analysis by Alexander Platt, an associate professor at the University of Kansas Law School, found that whistleblowers who retained counsel received substantially higher awards on average than those without, and of those who hired lawyers, a small group of repeat players—many of whom once worked at the SEC—was by far the most successful.