Washington DC - A federal judge has quashed subpoenas issued to the Federal Reserve, as part of a Department of Justice investigation linked to the cost of renovating the central bank's headquarters.

"A mountain of evidence suggests that the Government served these subpoenas on the Board to pressure its Chair into voting for lower interest rates or resigning," wrote Judge James Boasberg in a filing dated March 11, referring to Fed Chair Jerome Powell.

"The Court therefore finds that the subpoenas were issued for an improper purpose and will quash them."

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The documents were made public Friday.

US prosecutor Jeanine Pirro swiftly responded, saying the Trump administration would appeal the order, calling the decision "outrageous" and "the antithesis" of justice.

"This is the antithesis of American justice. Exonerating anyone without any records, without an investigation or question, is not how our criminal justice system works," Pirro told reporters.

"No one, folks, is above the law, and this outrageous decision will be appealed by the United States Department of Justice," she said.

President Donald Trump has repeatedly insulted Powell over the independent central bank's policies on setting the economy's key interest rate.

In January, Powell revealed that the Justice Department had launched a probe linked to cost overruns in renovations of the Fed's headquarters in Washington.

The investigation was unprecedented, and Powell responded by releasing a statement saying it was meant to intimidate him and the Fed.

"This is about whether the Fed will be able to continue to set interest rates based on evidence and economic conditions – or whether instead monetary policy will be directed by political pressure or intimidation," he said at the time.

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On Friday, Judge Boasberg's order was scathing in its criticism of Pirro's office.

"The Government has produced essentially zero evidence to suspect Chair Powell of a crime; indeed, its justifications are so thin and unsubstantiated that the Court can only conclude that they are pretextual."

The Fed did not comment on the issue when queried by AFP.