WASHINGTON — The chief ethics officer of the Environmental Protection Agency — the official whose main job is to help agency staffers obey government ethics laws — has been working behind the scenes to push for a series of independent investigations into possible improprieties by Scott Pruitt, the agency’s administrator, a letter sent this week says.

The letter is the first public acknowledgment that Kevin S. Minoli, who has frequently defended Mr. Pruitt’s actions since he took over the agency in February 2017, is now openly questioning whether Mr. Pruitt violated federal ethics rules.

The investigations recommended by Mr. Minoli include an examination of how Mr. Pruitt rented a $50-a-night condominium on Capitol Hill last year while he was being lobbied by J. Steven Hart, the spouse of the condo’s owner, according to a federal official with firsthand knowledge of the inquiries, who asked not to be named since the details of the investigation are intended to remain confidential.

In March, Mr. Minoli defended Mr. Pruitt’s lease for the condo, saying that Mr. Pruitt paid what appeared to be fair market value. He subsequently learned that Mr. Hart, who at the time was chairman of the lobbying firm Williams & Jensen, had repeatedly intervened with the E.P.A., and with Mr. Pruitt directly, on behalf of the lobbying clients that included Smithfield Foods and Coca-Cola.