The Justice Department is reportedly blocking Senate investigators from speaking with FBI officials who may provide first-hand testimony about Donald Trump's firing of ex-FBI chief James Comey.
It is the latest sign that Special Counsel Robert Mueller could be investigating the sacking as part of his Russia-related probe.
Mr Mueller is trying to determine if Mr Trump's campaign advisers colluded with the Russian government to influence the 2016 US election. That investigation also reportedly involves a probe into whether the President obstructed justice when he allegedly asked Mr Comey to drop an inquiry into former National Security Adviser Michael Flynn's possible ties to Russia.
Mr Flynn was dismissed from his role in February after it was revealed that he had misled Vice President Mike Pence about his interactions with a Russian official.
Multiple congressional committees are also investigating Russia's meddling in last year's presidential race and any improper interference with the FBI's investigation.
The heads of the Senate Judiciary Committee, 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, according to CNN.
But the Justice Department has refused, citing "the appointment of Robert S. Mueller III to serve" as special counsel, the media outlet reported.
"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.
The Justice Department's response has apparently irked the committee, CNN said.
It was reported last week that Mr Mueller hopes to speak with several of the President’s current and former top aides who were privy to episodes relevant to his investigation.
Mr Mueller is said to have alerted the White House that he wants to interview former press secretary Sean Spicer, ex-Chief of Staff Reince Priebus, and current communications director Hope Hicks, among others, according to the Washington Post.
Each of the advisers are alleged to have been involved or aware of internal discussions that have drawn the interest of Mr Mueller during his investigation, including the conversations that led to Mr Trump's firing of ex-FBI chief James Comey and the dismissal of former National Security Adviser Michael Flynn.