US Attorney-General Merrick Garland has named Jack Smith, a war crimes prosecutor, to serve as special counsel to oversee Justice Department investigations tied to Donald Trump involving the former president’s handling of sensitive documents and efforts to overturn the 2020 presidential election.

Attorney-General Garland’s announcement on Friday came three days after Mr Trump, a Republican, announced he would run for president again in 2024.

Attorney-General Garland said Mr Trump’s candidacy, as well as Democratic President Joe Biden’s stated intention to run for re-election, made the appointment of a special counsel necessary.

Special counsels are sometimes appointed to investigate politically sensitive cases and they do their jobs with a degree of independence from the Justice Department leadership.

“The pace of the investigations will not pause or flag under my watch,” Mr Smith said in a statement.

“I will exercise independent judgment and will move the investigations forward expeditiously and thoroughly to whatever outcome the facts and the law dictate.”

Attorney-General Garland said Mr Smith would oversee the investigation into Mr Trump’s handling of government documents after leaving the White House last year and the probe into attempts to interfere with the peaceful transfer of power following the 2020 election.

“Appointing a special counsel at this time is the right thing to do,” Attorney-General Garland said at a news conference.

Mr Trump, in a statement to Fox News, said he “won’t partake” in the special counsel’s investigations.

“For six years I have been going through this, and I am not going to go through it any more,” Mr Trump said.

President Biden, who appointed Attorney-General Garland, did not respond to shouted questions from reporters about the special counsel during his only public appearance of the day.

The White House was not involved in the decision to appoint Mr Smith, an official said, speaking on condition of anonymity.

This marks the second time in the span of five years that the Justice Department has appointed a special counsel to probe Mr Trump’s conduct.

Former FBI director Robert Mueller, appointed as a special counsel in 2017, documented contacts between Mr Trump’s 2016 presidential campaign and Russia, but found insufficient evidence to bring a charge of criminal conspiracy.

Mr Smith’s appointment comes at a time when FBI agents across the country have faced threats in the aftermath of the law enforcement agency’s seizure of thousands of government records, some marked as highly classified, during a court-approved August 8 search of Mr Trump’s Florida estate.

The search was part of a criminal investigation that Mr Trump has called politically motivated witch hunt. Investigators also are examining Trump for possible obstruction of the probe.

Mr Trump filed a civil lawsuit in an effort to delay the documents investigation and keep some records from investigators, claiming they are privileged.

The other investigation is a sprawling probe into a failed plot by Mr Trump’s allies to overturn the 2020 presidential election by submitting batches of phony slates of electors to the US National Archives and trying to stop Congress certifying President Biden’s election victory.

Mr Smith, a political independent, until recently served as the chief prosecutor for the special court in The Hague, tasked with prosecuting war crimes in Kosovo.

He previously oversaw the Justice Department’s public integrity section and worked as a federal and state prosecutor in New York.

The raft of criminal and civil state and federal investigations against Mr Trump also includes a civil lawsuit by New York state’s attorney-general involving his business practices.

– AAP