Laura Sweeney, a spokeswoman for the Justice Department, said the department’s Office of Professional Responsibility is investigating the Siegelman case.

Image Don Siegelman, left, a former Democratic governor of Alabama, and Ted Stevens, a former senator from Alaska. Credit... Left, Dave Martin/Associated Press, right, Gerald Herbert/Associated Press

But she pointed out that a panel of the United States Court of Appeals for the 11th Circuit recently upheld Mr. Siegelman’s conviction, and because he is appealing that ruling, “the department will continue to litigate this matter in the courts, not in the media.”

Mr. Kilborn wrote a letter to Mr. Holder on April 3 laying out several of his charges of misconduct by prosecutors. He said Ms. Stemler sent a letter to lawyers on both sides concerning accusations that emerged in the appeals process that jurors had exchanged improper e-mail messages during the trial.

The letter revealed a private communication between United States marshals and the judge in the case that Mr. Kilborn characterized as inappropriate. The letter from Ms. Stemler came so late in the process, he said, that it limited options for the defense.

Ms. Stemler’s letter stated that the communication between the judge and the marshals had no effect on the case, however, and that it was only being revealed “out of an abundance of caution.” Mr. Kilborn scoffed at that logic, saying that any private dealings with the judge should have been noted at the time.

In the interview, but not in the letter, he noted that William Welch, chief of the department’s public integrity section, and his principal deputy, Brenda Morris, were held in contempt in the Stevens case and had a measure of involvement in the Siegelman case, though he did not offer evidence of misdeeds in his case.

Mr. Siegelman’s conviction centered on a donation by the former chief executive of HealthSouth, Richard M. Scrushy, to help retire a campaign debt. Prosecutors said that the donation was a deal for Mr. Scrushy’s appointment to the state hospital licensing board; Mr. Siegelman said it was nothing of the sort.