How did sensitive government documents from President Joe Biden's time as vice-president of the United States end up in his garage?
That's one of the key questions a special counsel appointed by the nation's top law enforcement official will seek to answer in a probe announced on Thursday, local time, in Washington DC.
US Attorney-General Merrick Garland said it was "in the public interest" to assign veteran prosecutor Robert Hur to investigate Mr Biden's handling of two batches of Obama-era classified documents, recently discovered by the president's legal team.
One set, disclosed earlier this week, was found last November by Mr Biden's personal lawyers in a private office he used before launching his 2020 campaign.
The other set, revealed just days later, was being stored in the garage at his home in Wilmington, Delaware, next to his famous Corvette.
"By the way, my Corvette's in a locked garage. So, it's not like it's sitting out on the street," he told reporters on Thursday afternoon, local time, after being pressed on his storage choices.
Mr Biden vowed to cooperate "fully and completely" with the Department of Justice (DoJ) investigation.
"People know I take classified documents seriously," he said.
"I'm cooperating fully and completely with the Justice Department's review."
The White House has characterised the number of documents as "small" and said they were returned swiftly to National Archives and Records Administration (NARA).
However, the discovery has elicited a sense of deja vu in the American public after a messy legal fight between NARA and former president Donald Trump over a separate trove of classified materials.
It's also drawn accusations of hypocrisy from Republicans still furious about the FBI search of Mr Trump's Mar-a-Lago home to retrieve them, and the ensuing investigation.
Hold up, what is a special counsel?
A special counsel is a semi-independent federal prosecutor who operates outside the DoJ's regular chain of command.
Justice Department rules allow the attorney-general to appoint a special counsel to oversee criminal investigations in "extraordinary circumstances".
Such circumstances could include the need to avoid a perceived conflict of interest, or where it is in the public interest to bring in an outside prosecutor.
Mr Garland said Mr Hur's appointment underscored the DoJ's commitment to "independence and accountability in particularly sensitive matters, and to making decisions indisputably guided only by the facts and the law".
It also means there are now two special counsels looking into presidents' handling of government secrets.
In November, Mr Garland appointed another special counsel, Jack Smith, to oversee the investigation into documents found at Mr Trump's Florida residence.
Mr Smith, who previously investigated war crimes at The Hague, is also overseeing the criminal probe into Mr Trump's actions during the January 6 attack.
Sarah Krissoff, a former federal prosecutor in the US Attorney's Office for the Southern District of New York, said Mr Garland was "proceeding cautiously" by appointing a second special counsel.
"The attorney-general no doubt understands the political implications of this investigation into Biden's possession of classified information," she said.
"And I think, in order to avoid any appearance of impropriety here, he's appointed the special counsel to try to provide this independent voice to the investigation."
How do the Biden and Trump cases compare?
While on the surface the two investigations may bear some similarities, Brian Jacobs, who also served as a federal prosecutor in the Southern District of New York, warned against conflating them.
"They are night and day," he said.
"In the case involving Mar-a-Lago, based on the public reports, there were multiple requests by the government that former president Trump return the documents.
"There was an attestation signed by an attorney saying that all the documents had been returned. There was a subpoena. There was a search warrant that the Department of Justice executed."
Under the Presidential Records Act, presidential records must be sent to NARA at the end of a president's term, which Mr Trump refused to do.
More than a dozen boxes of White House materials were seized from Mar-a-Lago, which also operates as a members' only golf club.
Some of the documents found by the FBI were marked as "classified/TS/SCI", which is shorthand for "top secret/sensitive compartmented information".
The US government has three tiers for classifying sensitive information: confidential, secret and top secret.
Its highest level of security clearance is required to access sensitive, compartmented information and such documents are only meant to be stored and viewed in special secure facilities.
The documents found at Mr Biden's former office and his home date back to the Obama administration, when he was vice-president.
The White House has not disclosed what kind of information they included, nor their level of classification.
What the administration has said is that, on November 2, Mr Biden's lawyers found a "small number" of classified documents in a locked closet at the Penn Biden Center for Diplomacy and Global Engagement, a think tank in Washington.
They alerted NARA that day and returned the papers the next.
The Archives then referred the matter to the DoJ, which selected US attorney John Lausch to investigate.
According to the White House, the president's lawyers "immediately" told Mr Lausch about the second set of papers found in a storage space in his garage and an adjacent room on December 20.
Mr Lausch later recommended a special counsel be appointed to continue the probe.
The key difference, according to both Mr Jacobs and Ms Krissoff, is that the Biden administration self-reported the documents and voluntarily turned them over.
"The investigation into former president Trump is really an investigation into obstruction of justice," Ms Krissoff said.
"And what happened after the requests for those documents were made and the documents were identified as being in the former president's possession."
Could Biden be criminally prosecuted?
The question of whether a sitting president can face criminal prosecution has been hotly debated, including while Mr Trump was president, and there is no definitive answer.
While the newly appointed special counsel is authorised to prosecute any crimes arising from his investigation, or to refer the matter to relevant federal prosecutors, Mr Jacobs said in his view it was "extremely unlikely" charges would be laid against Mr Biden at this stage.
"The mishandling of classified information typically only gets prosecuted if there's some sort of plus factor showing that the mishandling was accompanied by wilfulness and bad intent," he said.
He suggested the "over-classification" of some government documents could also result in less-sensitive materials being inadvertently mishandled or misplaced.
Ms Krissoff agreed she did not see any basis for a criminal case against the president at this time.
"People make mistakes. They're humans," she said.
"The most important thing is to disclose those mistakes and be up-front about those mistakes.
"It is different matter, of course, to obstruct justice, which is a criminal offence."
Whatever happens next, most experts agree some of these problems could be avoided if the US government relied less on hard copies of documents.
"Electronic filing processes might frankly be more efficient than these boxes of papers that the presidents and former presidents are carting around," Ms Krissoff said.
