WASHINGTON — (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.