Immunity agreements offered to two of Hillary Clinton's top aides prevented the FBI from looking into the circumstances surrounding the use of BleachBit, a digital deletion tool, to destroy the former secretary of state's emails.

In a letter to Attorney General Loretta Lynch Wednesday, four Republican committee chairmen demanded to know why Cheryl Mills and Heather Samuelson, two witnesses who also served as Clinton's personal attorneys, were granted such expansive protections despite the FBI's awareness that they had participated in potentially illegal activities.

For example, the FBI agreed to limit its search to emails written after June 1, 2014, but before Feb. 1, 2015. By doing so, investigators were barred from looking at emails authored around the time Mills and David Kendall, Clinton's lead attorney, held a pair of conference calls with technology contractor Paul Combetta that immediately preceded his use of BleachBit to erase thousands of Clinton's emails.

The GOP lawmakers noted that, before the FBI signed off on the immunity deals, "it already knew of the conference calls between Secretary Clinton's attorneys and Mr. Combetta, his use of BleachBit and the resulting deletions, further casting doubt on why the FBI would enter into such a limited evidentiary scope of review with respect to the laptops."

FBI Director James Comey defended his decision to allow the immunity deals by arguing the protections extended only to the laptops Mills and Samuelson used to sort Clinton's emails before turning 30,000 over to the State Department.

Lawmakers expressed outrage last week following the discovery that the deals requested by Mills and Samuelson included a requirement to destroy their laptops after agents performed narrow searches on them.

Rep. Jason Chaffetz, chairman of the House Oversight Committee, Rep. Bob Goodlatte, chairmen of the House Judiciary Committee, Rep. Devin Nunes, chairman of the House Intelligence Committee, and Sen. Chuck Grassley, chairman of the Senate Judiciary Committee, complained in their letter that the Justice Department had severely limited their ability to review FBI materials from the Clinton email case.

For example, just a handful of lawmakers were permitted to view the records only at a specific location, accompanied by a limited staff and barred from taking pictures or notes.

"These onerous restrictions are not consistent with the high degree of transparency you and Director Comey promised to Congress," the chairmen wrote to Lynch.

Combetta, an employee at Platte River Networks, was also given an immunity deal in order to cooperate with the FBI. Combetta did not admit to destroying records that were already covered under congressional preservation orders until his third interview with the FBI.