Associate Special Counsel William Reukauf says in a letter to President Barack Obama that he's reviewed the Justice Department report and its findings were "reasonable."
Siegelman questioned the results Friday and said he did not believe an honest attempt was made to find the truth.
The whistleblower was Tamarah Grimes, a paralegal for the Montgomery team that prosecuted Siegelman and former HealthSouth CEO Richard Scrushy. She accused prosecutors of mismanagement and failure to report improper contact with jurors.
Siegelman was convicted in 2006 and sentenced to more than seven years in federal prison. He's free on bond while he appeals.