Story highlights The Senate judiciary committee wants to interview FBI officials as part of their probe

The congressional investigations investigating Russian interference are clashing with DOJ

(CNN) The Justice Department is preventing Senate investigators from interviewing two top FBI officials who could provide first-hand testimony over the firing of former FBI Director James Comey, the latest sign that Special Counsel Robert Mueller could be investigating the circumstances around the firing, officials tell CNN.

The previously undisclosed turf war comes as the Senate judiciary committee has not yet given assurances to the special counsel's office that it could have unfettered access to the transcript of the interview it conducted last week with the President's eldest son, Donald Trump Jr., saying that the full Senate must first authorize the release of the information to Mueller's team.

What appears to have irked the panel in particular is the refusal of the Justice Department to cooperate with a key part of its investigation. The leaders of the panel, Senate Judiciary Chairman Chuck Grassley and the ranking Democrat Dianne Feinstein, have repeatedly asked two senior FBI officials -- Carl Ghattas and James Rybicki -- to sit down for a transcribed interview to discuss the Comey firing as part of its inquiry into any improper interference with the FBI.

But the Justice Department has declined, citing "the appointment of Robert S. Mueller III to serve" as special counsel about Russian interference in the 2016 elections and "related matters."

"Under these circumstances and consistent with the Department's long-standing policy regarding the confidentiality and sensitivity of information relating to pending matters, the Department cannot make Mr. Ghattas nor Mr. Rybicki available for transcribed interviews at this time," according to a July letter signed by Acting Assistant Attorney General Samuel Ramer, which was reviewed by CNN.

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