The US attorney general, Merrick Garland, appointed a special counsel on Thursday to investigate Joe Biden’s retention of classified documents from his time as vice-president.

The documents were discovered to have been stored in his garage at his home in Delaware and in office space in Washington DC.

The move to name Robert Hur, a former Trump-appointed federal prosecutor and former top justice department official, was a rapid decision from Garland to insulate the department from possible accusations of political conflicts or interference.

Hur will be responsible for investigating the potential unauthorized removal and retention of classified documents at Biden’s home and his former thinktank, and will have the authority to prosecute any crimes resulting from the investigation, the order signed by Garland said.

“I will conduct the assigned investigation with fair, impartial and dispassionate judgement. I intend to follow the facts swiftly and thoroughly, without fear or favor, and will honor the trust placed in me to perform this service,” Hur said in a statement released by the justice department.

The decision to appoint a special counsel comes at a fraught moment for Garland, who only just named Jack Smith in November to serve as special counsel investigating Donald Trump’s unauthorized retention of national security material and his role in the January 6 Capitol attack.

Speaking at justice department headquarters in Washington, Garland said “extraordinary circumstances” – namely that the president, to whom he and the department reports, could yet become ensnared in the investigation – necessitated an independent prosecutor to oversee the inquiry.

The announcement comes amid growing scrutiny of whether Biden was involved in taking the documents to either his Delaware home or the office space at the University of Pennsylvania’s Biden Center for Diplomacy in Washington, where he was an honorary professor until 2019.

Special counsels are prosecutors with more independence than other federal prosecutors, who can be installed for high-profile investigations when there are conflicts of interest or the appearance of such conflicts, though they ultimately answer to the attorney general.

The Biden special counsel will examine approximately 10 classified documents found at the thinkthank, which included US intelligence memos and some materials marked as Top Secret/Secret Compartmented Information, and an unconfirmed additional number of classified documents that were in the garage and a storage space nearby.

From the information released so far, there are major differences between the Biden case and that involving Trump, who retained hundreds of classified documents and only partially complied with a grand jury subpoena that led to the FBI search of Mar-a-Lago last August.

By contrast, Biden and his lawyers proactively returned the classified documents dating back to the Obama administration when he was vice-president to the government as soon as they were discovered. Biden was also not responding to a grand jury subpoena.

Biden’s personal lawyers found the first set of documents at the Penn Biden Center on 2 November and alerted the National Archives and the justice department. The archives then issued a formal referral, leading Garland to task the Trump-appointed US attorney John Lausch to conduct a review.

Biden’s lawyers then found additional classified documents in Delaware on 20 December. On 5 January, Lausch recommended that Garland appoint a special counsel to conduct an investigation. Lausch added it could not be him, since he was due to return to private practice, according to a source familiar with the matter.

The revelation about the new documents came hours after the White House press secretary, Karine Jean-Pierre, said the White House was committed to handling the matter in the “right way”, pointing to Biden’s personal attorneys’ immediate notification of the National Archives.

“As my colleagues in the counsel have stated and said to all of you yesterday, this is an ongoing process under the review of the Department of Justice. So we are going to be limited on what we can say here,” Jean-Pierre said.

Before the special counsel appointment, Biden told reporters: “I’m going to get a chance to speak on all this, God willing, soon. People know I take classified documents and classified material seriously. I also said we’re cooperating fully and completely with the justice department’s review.”

The top Republican on the House intelligence committee has since requested that the US intelligence community conduct a “damage assessment” to assess the impact of their storage in an unauthorized location.

In a statement on his Truth Social platform shortly before Garland spoke on Thursday, Trump said Garland should “immediately end special counsel investigation into anything related to me because I did everything right, and appoint a special counsel to investigate Joe Biden who hates Biden as much as Jack Smith hates me”.

According to a justice department biography, Hur’s prosecutorial career has included cases including gang violence, drug trafficking and domestic terrorism, as well as white-collar crime such as fraud, tax offenses, intellectual property theft and public corruption.

Hur graduated from Harvard College and received his law degree from Stanford. He clerked for Chief Justice William Rehnquist before working for the current FBI director, Christopher Wray, when he ran the justice department’s criminal division during the George W Bush administration.

Most recently, he was a partner at the highly esteemed law firm Gibson, Dunn and Crutcher after he left the justice department at the end of the Trump administration, where he served as the principal associate deputy attorney general, the top adviser role to the deputy attorney general.