A spokesman for JPMorgan, when asked about the exchange with the executives, disputed the account, saying the bank does not trade ethanol credits for a profit in the way it trades other securities, but is registered to deal in credits through its energy business. From time to time, the spokesman, Brian J. Marchiony, said in a statement that the bank also purchased credits “on behalf of clients who need to fulfill their E.P.A.-mandated obligations,” though it had not done so in the past year.

But other market participants, including Thomas D. O’Malley, chairman of PBF Energy in Parsippany, N.J., identified JPMorgan Chase and other financial institutions as being active sellers of the credits this year. He said the institutions had helped transform an environmental program into a profit machine, contributing to the market frenzy this year. “These things were designed to monitor the inclusion of ethanol in the gasoline pool,” Mr. O’Malley said. “They weren’t designed to become a speculative item. For the life of me I can’t see the justification for it.”

While banks are by no means the largest player in ethanol credits, Wall Street’s activity in this market reflects a larger effort by financial institutions to exert their influence over loosely regulated markets for basic commodities, from aluminum to oil. The opacity of the ethanol credit market makes it difficult to determine the extent to which large financial actors have profited.

The banks say they have far less influence in the market than others are suggesting, and are doing nothing wrong. But the activities, while legal, could have consequences for consumers. In the end, energy analysts say, the outcome will be felt at the gas pump — as the higher cost of the ethanol credits gets tacked onto the price of a gallon of gasoline. (The credits, which cost 7 cents each in January, peaked at $1.43 in July, and now are trading for 60 cents.)

The Valero Energy Corporation, a refiner that owns thousands of gas stations, says the squeeze in ethanol credits might cost it $800 million. PBF Energy, also a refiner, puts its bill at about $200 million. A review by The Times of a federal registry of nearly 1,500 businesses and individuals in the renewable fuel market found big Wall Street banks as well as a handful of people with troubled legal histories among the participants. Several high-profile cases of fraud have emerged.