Washington DC - Nine FBI agents sued the Justice Department on Tuesday, seeking to block its efforts to collect information on agents involved in investigating President Donald Trump and the January 6, 2021, Capitol riot by his supporters.

The FBI agents who filed the complaint against Trump's acting attorney general, James McHenry, were not identified for their own protection in the complaint filed in a federal district court in Washington.

The lawsuit follows reports that acting deputy attorney general Emil Bove, Trump's former personal lawyer, had asked FBI employees to respond by Tuesday to a questionnaire outlining the work they did on the sprawling Capitol riot investigation.

Trump, on his first day in the White House, pardoned more than 1,500 of his supporters who stormed Congress on January 6, 2021, in a bid to block the certification of Democrat Joe Biden's 2020 election victory.

In their complaint, the FBI agents said the effort to collect information on employees who participated in the Capitol riot investigation was part of a "purge" orchestrated by Trump as "politically motivated retribution."

"This directive is unlawful and retaliatory," they said in asking a federal judge to block the compilation of a list of FBI employees who worked on the Capitol riot cases and the case brought against then-former-president Trump for stashing top secret documents at his Mar-a-Lago home in Florida.

The FBI agents noted that the Justice Department had fired a number of officials last week who were involved in the prosecutions of Trump for seeking to overturn the 2020 election results and the documents case.

Both cases were dismissed after Trump won the presidency.

The FBI agents said they had been instructed to fill out a survey that would "identify their specific role in the Jan. 6 and Mar-a-Lago cases."

"Plaintiffs assert that the purpose for this list is to identify agents to be terminated or to suffer other adverse employment action," they said.

They expressed fears the list could be published by Trump allies, "placing themselves and their families in immediate danger of retribution by the now pardoned and at-large Jan. 6 convicted felons."

The nine FBI agents said they represent at least 6,000 current or former FBI employees who participated in some manner in the investigation of "crimes and abuses of power by Donald Trump, or by those acting at his behest."

A copy of the questionnaire sent to FBI employees was attached to the complaint.

It asks them whether they took part in any arrests, evidence collection, witness interviews, or other activities related to the Capitol riot probe.

FBI director Christopher Wray resigned following Trump's reelection and the president has named Kash Patel, a staunch loyalist, to head the top US law enforcement agency.

At his Senate confirmation hearing, Patel was asked if he was aware of any plans to punish FBI agents involved in the Trump investigations.

"I am not aware of that," he said, adding that "all FBI employees will be protected against political retribution."