The US Justice Department is investigating an alleged bribery scheme involving campaign donations to secure a presidential pardon, a court filing showed Tuesday.

The document, which discusses the legality of searching communications and electronic devices of individuals, including attorneys, is highly redacted, with all identifying information blacked out.

But it refers to a "secret lobbying scheme" directed at "senior White House officials" to gain a presidential "pardon or reprieve of sentence" for an unnamed individual.

The scheme, under investigation since at least August, appears to have involved lobbyists and lawyers, a well-heeled donor to political campaigns, and a man or woman who is or was in prison and is hoping for presidential intervention.

The filing indicates that the lobbyists and lawyers contacted White House officials requesting a presidential pardon or reprieve, citing the "past substantial campaign contributions" and "anticipated future substantial political contributions" from a donor.

It suggests that the donor is making the offer on behalf of the person seeking clemency.

The document does not indicate when the actions involved took place, and, in the sections not redacted, there is no reference to Trump or his campaign.

But the filing was revealed amid speculation that, with six weeks left in the White House after losing the November 3 election, President Donald Trump is preparing to grant executive clemency to more people, after pardoning his former national security advisor Michael Flynn last Wednesday.

Trump tweeted late Tuesday, "Pardon investigation is Fake News!"

Trump has granted pardons or sentence reductions to several political allies, including campaign consultant Roger Stone, controversial former Arizona sheriff Joe Arpaio and Republican activist Dinesh D'Souza.

The New York Times reported that Trump has discussed granting preemptive pardons to his lawyer, Rudy Giuliani, his three oldest children and his son-in-law Jared Kushner.

There is also public discussion of Trump issuing a pardon for himself, for any crimes he might be charged with related to his time in office -- though the legality of that has never been tested.

"A self-pardon would be a fitting abuse to end Trump's presidency. It would also be corrupt, illegitimate, and void," said Democratic congressman Adam Schiff in a tweet Tuesday.