WASHINGTON — A billionaire Wall Street commodities investor has been drawn into a court fight against one of the nation’s most influential anti-Iran advocacy groups, a case in which the Obama administration has introduced a sense of intrigue by its recent claim that, somehow, the group possesses important government secrets.

The fight stems from a defamation lawsuit by Victor Restis, a Greek businessman whom the group, United Against Nuclear Iran, accused last year of doing business with Tehran. The group has undertaken several such “name and shame” campaigns against companies around the world and has persuaded many of them to stop doing business in Iran. In turn, Mr. Restis has drawn the billionaire philanthropist Thomas S. Kaplan into the case, asking a federal judge to order him to turn over documents that he suspects are related to the case.

The lawsuit has taken on larger importance as it threatens to reveal government secrets. It also could make public the inner workings of United Against Nuclear Iran, which is run by former United States government officials, has lobbied for tough sanctions against Iran and has helped write legislation on Capitol Hill.

The Obama administration has temporarily blocked release of the group’s internal documents, including its emails and donor lists, saying the records are likely to contain law enforcement secrets. It is not clear how the group obtained the documents, but the Justice Department is concerned enough about their release that government lawyers are quietly mediating settlement talks to try to end the lawsuit, according to a federal law enforcement official who was briefed on the case and was not authorized to discuss confidential matters.