WASHINGTON — For more than a year, prominent former American officials have been giving well-paid speeches in support of an Iranian opposition group that is fighting to reverse its 15-year-old designation by the State Department as a terrorist organization.

Now the Treasury Department appears to have begun an inquiry to see whether the speaking fees were being paid by the group, the Mujahedeen Khalq, or People’s Mujahedeen, known as the M.E.K. Americans are prohibited by law from doing business with designated terrorist groups.

Edward G. Rendell, the former Democratic governor of Pennsylvania and an outspoken supporter of the M.E.K., said on Monday that William Morris Endeavor, which handles his speaking engagements, received a subpoena last week seeking information on fees he had received for M.E.K.-related speeches.

The Treasury Department declined to comment on whether it is conducting an investigation. But the subpoena to Mr. Rendell, earlier reported by The Washington Times, raises the possibility that a long list of former officials who have accepted fees to speak on behalf of the M.E.K., including former directors of the Central Intelligence Agency and the Federal Bureau of Investigation, could come under scrutiny for any payments traced to the group.