(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.

In late August, the leaders of the committee modified their request, hoping to reach an accommodation with the department.

In a letter to Deputy Attorney General Rod Rosenstein, Grassley and Feinstein said investigators would "limit the scope" of their questions to avoid the pending Mueller investigation and instead focus primarily on "independent recollections, as fact witnesses, of events that occurred before and including Director Comey's removal."

The committee, however, has not been able to interview those witnesses, despite asking that to happen by September 1.

"Thus far, we've not received cooperation from DOJ and the special counsel's office in scheduling those interviews voluntarily," George Hartmann, a spokesman for Grassley, said in an email.

The spokesman said there are "ongoing informal discussions" with the Justice Department to schedule those interviews but the department has "yet to produce any results."

A Justice Department spokeswoman said the matter is a "question for the special counsel." A spokesperson for the special counsel declined to comment.

The refusal to allow the Senate panel to interview those two witnesses could be a sign that Mueller is reviewing whether President Donald Trump acted properly in his firing of Comey. Critics have contended that the President may have been trying to interfere with Comey's criminal investigation into potential Russia collusion with the Trump campaign, something the White House has furiously denied.

The fight over witnesses comes as Mueller's investigation is colliding with several inquiries on Capitol Hill, which are competing for witnesses and testimony over possible Trump campaign links to Russia.

The Senate judiciary committee has not yet said Mueller can have access to the Trump Jr. transcript, citing a standing rule of the Senate that requires approval from the full chamber to provide any information to the executive branch to use in a pending investigation.

"Both the judiciary committee and the special counsel are engaged in important investigations, and we are committed to being as cooperative with Mueller's office as we can, which is why we've been in ongoing communication with Mueller's team," Hartmann said.

The challenges in obtaining the transcript is one reason why Mueller's team has communicated to some on Capitol Hill that it would like key witnesses to testify in public, rather than behind closed doors.

Feinstein told CNN this week that "come hell or high water" Trump Jr. would testify in public before her committee, largely over his June 2016 meeting with Russian operatives where he was promised dirt on the Clinton campaign. But Grassley so far has been non-committal about a public session.

"I'm going to talk to Sen. Feinstein about that, and make a decision after we talk to her," Grassley told CNN Tuesday. "But don't forget, we've got several staff interviews. We've got to get the transcript. We've got to make sure that the people that gave the transcript get a chance to check it for accuracy."