Merrick Garland, the US attorney general, has appointed a special counsel to determine whether Donald Trump, the former president, should face criminal charges stemming from investigations into his alleged mishandling of national security materials and his role in the January 6 attack on the US Capitol.

The politically explosive move comes just three days after Trump announced he is running for the White House yet again, despite a disappointing Republican performance in the midterm elections, especially among candidates backed by the ex-president.

“Based on recent developments, including the former president’s announcement that he is a candidate for president in the next election, and the sitting president’s stated intention to be a candidate as well, I have concluded that it is in the public interest to appoint a special counsel,” Garland told a press conference on Friday.

Garland named Jack Smith, a veteran prosecutor, to the post, which will deal with justice department investigations into Trump’s attempt to subvert the 2020 presidential election victory for Joe Biden, and also the discovery of confidential documents at Trump’s Mar-a-Lago residence in Florida.

Trump attacked the appointment within hours, in an interview with Fox News’s digital arm.

“For six years I have been going through this, and I am not going to go through it any more,” Trump said. “It is not acceptable. It is so unfair. It is so political.”

The appointment of a special counsel reflects the sensitivity of the justice department overseeing the two most hazardous criminal investigations into Trump, and an increased possibility of charges being brought over either matter.

Special counsels are semi-independent prosecutors who can be installed for high-profile investigations when there are conflicts of interest, or the appearance of such conflicts, and provide a mechanism for the justice department to insulate itself from political considerations in charging decisions.

“I strongly believe that the normal processes of this department can handle all investigations with integrity,” Garland said. “And I also believe that appointing a special counsel at this time is the right thing to do. The extraordinary circumstances presented here demand it.”

The attorney general added: “I will ensure that the special counsel receives the resources to conduct this work quickly and completely. Given the work done to date and Mr Smith’s prosecutorial experience, I am confident that this appointment will not slow the completion of these investigations.”

Smith, a graduate of Harvard law school, from 2010 to 2015 served as the chief of the public integrity section at the justice department, which handles government corruption investigations, a role not dissimilar to his new position as special counsel.

Since 2018, he has been a special prosecutor to The Hague investigating war crimes in Kosovo, having joined the international criminal court from the US attorney’s office for the eastern district of New York in Brooklyn, where he helped prosecute a police brutality case that drew national attention.

During his time at the justice department in Washington, Smith oversaw the corruption cases against former Virginia governor Bob McDonnell, ex-Arizona congressman Rick Renzi and New York assembly speaker Sheldon Silver, though convictions against McDonnell and Silver were later overturned.

He oversaw the prosecution of a CIA agent for disclosing national defense information and obstructing justice – crimes that echo potential charges against Trump.

And Smith has also investigated Trump before, in the 1970s, over potential fraud charges during his tenure as a prosecutor in New York. The roughly six-month investigation ultimately yielded no charges, after which Trump complained about the investigation.

Politico reported that Smith was registered to vote as a political independent, not a Democrat or a Republican.

In a statement released by the justice department, Smith said: “I intend to conduct the assigned investigations, and any prosecutions that may result from them, independently and in the best traditions of the Department of Justice.

“The pace of the investigations will not pause or flag under my watch. I will exercise independent judgment and will move the investigations forward expeditiously and thoroughly to whatever outcome the facts and the law dictate.”

The appointment of a special counsel will be a familiar dynamic for Trump, who was the subject of Robert Mueller’s investigation shortly after he took office, examining ties between his 2016 presidential campaign and Russia. Later, Trump’s attorney general, Bill Barr, appointed special counsel John Durham to investigate allegations of FBI impropriety in the Russia investigation.

Trump has already spent months since the FBI seized 103 documents marked classified from Mar-a-Lago accusing the justice department under Joe Biden of pursuing him for political reasons – a tension likely to become more biting as the 2024 election draws nearer.

It was to allay those concerns, Garland said at the news conference, that he chose to appoint Smith to run the investigations. “Appointing a special counsel at this time is the right thing to do,” Garland said. “The extraordinary circumstances presented here demand it.”

The appointment of a special counsel could indicate that the justice department has already accumulated substantial evidence of potential criminality by Trump and his allies. Barbara McQuade, a University of Michigan law school professor and former US attorney, said: “One thing that is significant is this suggests that they think there’s a very real possibility of charges. If they were going to close the case, it would be closed by now.”

But some criticised the move as inadvertently buying Trump time and allowing an over-cautious Garland to duck responsibility. Jill Wine-Banks, a legal analyst and former Watergate prosecutor, tweeted: “Garland has named a Special Counsel to investigate Trump #MAL and parts of Jan6. I think it’s a waste of time and money, insults the prosecutors at DOJ and gains nothing. No Trump supporter will see anyone as independent or fair to Trump.”

The Lincoln Project, an anti-Trump group, tweeted: “The announcement of a special counsel to investigate Trump in light of the abundance of clear and convincing evidence of his crimes unfortunately delays accountability. However, justice will come eventually & he will not be able to evade the consequences of his actions forever.”

The White House press secretary, Karine Jean-Pierre, said Biden had not been given any advance notice of Garland’s announcement. “No, he was not aware, we were not aware,” she said at a delayed press briefing. “The department of justice makes decisions about criminal investigations independently. We are not involved.”

Jean-Pierre added: “We were not given advance notice. We were not aware of this investigation.”