The credits can also be resold, a commonplace activity in the arena of corporate compliance with federal environmental rules.

The problem is that at least three companies were selling bogus credits without producing any biodiesel at all, the E.P.A. has said in announcements over the last year.

Now no one is certain how many of the credits are real. So far, more than $100 million in fraudulent credits have been identified, the refining industry estimates. That amounts to roughly 5 percent of the credits issued since 2009, but the percentage could rise as current investigations of other producers progress.

The credits are easier to counterfeit than hundred-dollar bills. Known as “renewable identification numbers,” or RINs, the 38-digit credits have no physical form and are traded electronically. Exxon Mobil, Marathon and Sunoco are among the many big companies that have bought bogus credits.

Last April, the E.P.A. announced settlements with oil companies that had submitted invalid RINs sold by two of the three fraudulent producers. The penalties, amounting to 30 cents per gallon of biofuel, ranged from a few thousand dollars to a maximum of $350,000. The agency also required such companies to buy legitimate credits to replace them.