(CNN) Special counsel Robert Mueller's office missed a deadline to respond to a congressional inquiry about whether he is preventing secretary of the state nominee Mike Pompeo from disclosing communications with President Donald Trump about the Russia investigation, according to a source familiar with the matter.

After Pompeo declined to answer certain questions about the Russia probe during his confirmation hearing last week at the Senate Foreign Relations Committee, the committee's top Democrat, Sen. Bob Menendez of New Jersey, sent a letter asking Mueller if there's "any prohibition on Director Pompeo" from answering his questions. After setting a Friday deadline, Mueller's office has not yet responded, the source said.

The push comes as Pompeo is facing stiff opposition from Democrats , who appear to have the votes to deny him majority support in committee with the help of opposition from GOP Sen. Rand Paul of Kentucky. Pompeo still may get confirmed if the GOP unites on the floor and at least one Democrat defects to support his nomination as the top US diplomat.

The Menendez letter came after Pompeo sidestepped questions about a meeting Trump reportedly had last March when the President reportedly complained about then-FBI Director James Comey, and urged his top intelligence official, Dan Coats, to persuade Comey to back off the investigation into the former national security adviser, Michael Flynn. Pompeo was in attendance at the meeting, according to The Washington Post.

During his testimony last week, Pompeo would not discuss what Trump said to him and Coats at the meeting.

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