The US Justice Department is investigating a potential crime related to funnelling money to the White House in exchange for a presidential pardon, according to court documents unsealed in federal court.

US District Judge Beryl Howell has released a heavily redacted order that described what she called a “bribery-for-pardon” investigation.

About half of the 18-page document was blacked out, with the publicly-available version providing few details of the alleged scheme, and naming none of the people potentially involved.

According to the order unsealed by Judge Howell on Tuesday (local time), federal prosecutors in Washington said they had obtained evidence of a bribery scheme in which someone “would offer a substantial political contribution in exchange for a presidential pardon or reprieve of sentence”.

The Justice Department had to ask Judge Howell’s permission to view certain electronic communications between a lawyer and clients, who were not identified.

Judge Howell granted the request in August, saying attorney-client privilege did not apply in that instance.

The documents made public do not name the individuals involved or President Donald Trump. Nor do they indicate if any White House officials had knowledge of the scheme.

“No government official was or is currently a subject or target of the investigation disclosed in this filing,” a Department of Justice official said.

Prosecutors had said they planned to “confront” three unnamed individuals with the communications and finish their investigation.

Presidents enjoy wide latitude under the US Constitution in pardoning people convicted of federal crimes.

Last week, Mr Trump pardoned his former national security adviser Michael Flynn.

General Flynn had twice pleaded guilty to lying to the FBI during the investigation into Russian meddling in the 2016 presidential election.

That was the first of what is expected to be a string of pardons in Mr Trump’s final weeks in the White House.

-with AAP