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In addition to the Exxon stations, Mr. Mamo’s companies also supply gasoline to 34 other gas stations in the District, or 60 percent of the 107 gas stations in the city. His holdings also include a host of other stations around the D.C. area.

AAA Mid-Atlantic calculates that, as of Monday, the average cost for a gallon of unleaded gas in the District was $3.87 — well above the regional average of $3.64 a gallon.

“It’ some of the most expensive gas on the East Coast,” spokesman John B. Townsend II said, noting that the cause was the way distributors control the prices in the city.

Mr. Nathan has investigated Mr. Mamo’s company before, launching and later dropping a probe into possible antitrust violations, and didn’t sound likely to give up when faced with the latest legal hurdle.

“We are reviewing the decision and considering all avenues of relief, and we will continue to seek ways to end unlawful supply restrictions, increase competition, and bring down retail prices at the pump for consumers,” said Mr. Nathan’s spokesman, Ted Gest.

Ms. Cheh plans to introduce emergency legislation in June that would give the attorney general the authority to bring a legal claim against the company. If the bill is adopted, the hope is the attorney general could then refile the lawsuit and get a second round in court.