The letter comes as Republicans are pivoting to investigating Obama-era officials and scandals as they look to put Mueller's probe into Russian election interference and the Trump campaign behind them.

Grassley, who was previously chairman of the Judiciary Committee, sent a flurry of letters to the Justice Department during the previous Congress requesting information on the FBI's handling of the Clinton probe and a controversial research dossier compiled on then-candidate Trump.

Grassley requested a briefing on the information and sent a follow-up last year with questions about the annex, but the GOP senators noted in their letter Tuesday that the Justice Department initially refused to brief the panel, citing Mueller's investigation.

GOP senators noted on Tuesday that an annex, which was not released publicly because it's classified, included in the Justice Department inspector general report on the Clinton investigation "raises significant issues associated with the FBI’s failure to review certain highly classified information.""We are reissuing the attached classified letter regarding the important questions raised by the appendix and reiterating our request for a classified briefing on the subject," Grassley, Graham and Johnson wrote in their letter Tuesday.