Classified files have been found at ex-US Vice-President Mike Pence's home in the latest discovery of secret papers at the homes of officials who have served in the top ranks of government.
The documents, discovered by a lawyer last week for Mr Pence at his Indiana home, have been handed over to the FBI.
Investigators are already looking into President Joe Biden and ex-President Donald Trump's possession of files.
Mr Trump faces a criminal investigation for allegedly mishandling papers.
Representatives for Mr Pence sent a letter to the National Archives alerting them to the documents.
The FBI came to the former vice-president's home to collect the documents, bypassing "standard procedures" and requesting "direct possession" of them, lawyers added in a separate letter.
Under the Presidential Records Act, White House records are supposed to go to the National Archives once an administration ends. Regulations require such files to be stored securely.
A "small number of documents bearing classified markings" were "inadvertently boxed and transported" to Mr Pence's home at the end of Mr Trump's presidency, his lawyer wrote in a letter shared with US media.
The latest development emerged after Mr Pence sought legal help from specialists in handling classified documents "out of an abundance of caution".
He asked for help "after it became public that documents with classified markings were found in President Joe Biden's Wilmington residence", the letter read.
Lawyers found "a small number of documents that could potentially contain sensitive or classified information", which were locked by the former vice-president in a safe.
An aide to Mr Pence told CBS News, the BBC's US partner, the documents were stored in boxes in an insecure area of Mr Pence's home. The aide said they were taped shut.
According to US media, the documents are believed to have first been taken to Mr Pence's home in Virginia before later being sent to Indiana.
After the letter became public, Mr Trump came to Mr Pence's defence, taking to his Truth Social social media platform to say that he is "an innocent man".
"He never did anything knowingly dishonest in his life," Mr Trump wrote. "Leave him alone!!!"
Mr Pence had repeatedly said over the last months that he did not believe he was in possession of classified documents.
Earlier this month, he told CBS that he was confident reviews of documents in his home were done "in a thorough and careful way".
Mr Biden previously said he had "no regrets" over not going public before the midterm elections with the news that classified documents had been discovered in his private office.
Six more classified files were found during a 13-hour search of President Biden's home in Delaware on Friday, his lawyer Bob Bauer said in a statement on Saturday.
The documents unearthed so far are believed to be related to Mr Biden's eight-year tenure as vice-president under former President Barack Obama.
Mr Biden offered access "to his home to allow DoJ [the Department of Justice] to conduct a search of the entire premises for potential vice-presidential records and potential classified material", Mr Bauer added.
Earlier this month, Mr Biden's lawyers said a first batch of classified documents had been found on 2 November at the Penn Biden Center, a think tank that the president founded in Washington DC.
A second batch of records was found on 20 December in the garage at his Wilmington home, while another document was found in a storage space at the house on 12 January, his lawyers said.
Representatives for former Presidents Obama and George W Bush told Reuters on Tuesday that their administrations had turned over all documents to the National Archives after leaving office.
The discoveries at the homes of Mr Pence and Mr Biden come as Mr Trump faces a special counsel inquiry over his alleged mishandling of documents.
Hundred of classified records were found at Mr Trump's Florida Mar-a-Lago residence - Mr Trump and his lawyers resisted handing over the documents until the FBI raided the Florida holiday home last August.
He denied any wrongdoing, alleging that President Biden was being treated more favourably by the FBI.
Document discovery timeline
- 2 November 2022 - First batch of classified documents found at the Penn Biden Center, a think tank that President Joe Biden founded in Washington DC
- 20 December - Second batch of records found in the garage of Joe Biden's Wilmington home
- 12 January - Document found in a storage space at Mr Biden's Wilmington home
- 19 January - FBI agents come to Mike Pence's Indiana home to collect files
- 20 January - Department of Justice investigators discover six more classified documents during a 13-hour search of Biden's home in Delaware.