The District’s attorney general said Wednesday that his office is investigating the city’s largest owner of gas stations for potential antitrust violations.

Attorney General Irvin B. Nathan said that his office was looking into allegations that Capitol Petroleum Group of Springfield was engaging in anticompetitive practices that could be leading to artificially high gasoline prices.

“I can tell you that this is an active investigation,” he said. “It is one of the priorities of the antitrust part of our office.”

The company is a regional behemoth, netting $778 million in revenue in 2010, according to its Web site. It “owns, operates or supplies” 164 stations in the D.C. area and another 71 stations in New York City.

Its majority owner, Eyob “Joe” Mamo, has built the company over the course of nearly 25 years, buying up dozens of service stations in the region. A February Washington City Paper profile of Mamo said that the Ethiopian immigrant owned “nearly half” of the gas stations in the city limits.

”He’s occasionally been painted as a rapacious tycoon, and sometimes been cast as an exemplar of the American Dream. In fact, Mamo may be a bit of both,” Christine MacDonald wrote in the profile.

Mamo is a frequent donor to city political campaigns. According to campaign finance records, Mamo, his family members and his companies have donated more than $40,000 to D.C. candidates in the past decade — including $12,000 to former mayor Adrian M. Fenty’s re-election campaign.

The investigation was disclosed Wednesday at a news conference held by the man who beat Fenty (D), Vincent C. Gray (D). Nathan declined to discuss details of the investigation, but he said that his office will conduct interviews and issue demands for information on CPG’s business practices. Should a court find violations, he said, it could order the company to refrain from certain practices or sell off some of its stations.

Mamo could not be immediately reached for comment.

”Everyone knows that gasoline prices are high,” Nathan said, “and we’re going to do everything we can to bring them down and be sure that it is a competitive market.”