US President Joe Biden is under investigation after classified documents were found in two locations, including his home garage.

The appointment of a special counsel by Attorney General Merrick Garland has poured cold water over the president's run of political successes in the past few months and drawn comparisons to his predecessor and possible future Republican presidential candidate Donald Trump.

Although the comparisons are limited, the turn of events has cast a shadow of ambiguity over a repeat run for office by the 80-year-old president in 2024.

What documents have been found so far?

The first batch of documents was discovered by Biden's own legal team at his former Washington think-tank office on November 2, just before the mid-term elections.

The documents date back to former President Barack Obama's time in office when Biden served as vice president.

Biden's special counsel Richard Sauber said on January 10 that "a small number of documents with classified markings" had been found in a locked closet at the Penn Biden Center, a think-tank affiliated with the University of Pennsylvania.

In response to the first discovery, Biden said he had been "surprised" to learn the documents had been found in his old office but said he did not know what was in them.

Things became trickier for Biden after the second discovery of documents, this time at his residence in Wilmington Delaware.

Sauber said another "small number" of documents had been found in the president's garage, with one more file found in an adjacent room. The documents also dated back to Biden's time as vice president.

In both instances, Biden's legal team found the classified documents and "immediately notified" the Department of Justice, according to Sauber.

What are the next steps in the investigation?

Attorney General Garland appointed Robert Hur to act as a quasi-independent prosecutor. Hur served as the top federal prosecutor in Maryland under Trump.

Garland said Hur will examine "whether any person or entity violated the law."

"This appointment underscores for the public the department's commitment to both independence and accountability in particularly sensitive matters, and to making decisions indisputably guided only by the facts and the law," Garland told a news conference.

"We are confident that a thorough review will show that these documents were inadvertently misplaced, and the president and his lawyers acted promptly upon discovery of this mistake," Sauber said in a statement.

However, the political fallout will likely be greater than any legal implications for Biden due to the Justice Department's long-standing policy of not bringing criminal charges against sitting presidents.

Why is this a big deal?

The passing resemblance of the investigation into Biden's handling of classified documents and Trump's alleged obstruction of investigators who were trying to get their hands on the vast tranche of classified documents being stashed at his Mar-o-Lago home may have a couple of major consequences.

Firstly, it may bring a second Biden presidential run into question for Democrats even before he formally announces his intention to run again, which some expect will not happen until after March.

The possibility of a rematch between Biden and the 76-year-old Trump in 2024, but this time with both candidates under federal investigations, has triggered calls for space to be made for a new generation of leaders.

The second issue is the impact the revelations may have on the investigation into Trump himself.

"I don't see how a criminal case goes forward at this point,'' said John Bolton, a former national security advisor to Trump and erstwhile ardent critic of the former president.

"I just think it's such a cloud over the prosecution," he was quoted as saying by The Associated Press.

However, the investigation into Trump is of a different nature due to his alleged attempts to obstruct a federal investigation. The former president is also facing several other probes, including one looking into the January 6 Capitol insurrection.

What have the reactions been?

Biden has said he is cooperating fully with the investigation while defending his reputation, saying "people know I take classified documents and classified material seriously."

But Democrats admitted the situation makes things difficult. "No one's going to say this is helpful," veteran Democratic strategist James Carville was quoted as saying by the AP. "It's pretty evident that's not the case.''

Trump took an expectedly different approach to the discoveries, claiming that while his actions were without fault, Biden's alleged improper storage of classified documents was "bad."

''When all of these documents started coming out and Biden had them, it really changed the complexion and the intensity that they were showing to me because, you know, what they did is — I don't say far worse, I did nothing wrong — what they did is not good. What they did is bad," Trump told conservative talk radio host Mark Levin.

Republicans have also criticized the appointment of a special counsel, arguing that their own lawmakers should be given the chance to carry out an investigation.

"When special counsels are appointed, it limits our ability to do some of the oversight investigations that we want to do," Republican Representative James Comer, who is set to head up the House Oversight Committee, said.

But perhaps what is most important to both Democrats and Republicans is whether the general public distinguishes between the two cases.

"It's all the difference in the world between having something you don't know you have and having something you know you have and aren't supposed to have," veteran Democrat strategist James Carville said. "Is that going to get lost among a third of the country? Probably so."

This article was written in part with material from The Associated Press and Reuters.

Edited by: Rebecca Staudenmaier