NEW YORK, United States (AP) — Documents with classified markings were discovered in former Vice-President Mike Pence’s Indiana residence last week, his lawyer says, the latest in a string of recoveries of confidential information from the homes of current and former top US officials.

The records, which were taken into FBI custody, “appear to be a small number of documents bearing classified markings that were inadvertently boxed and transported to the personal home of the former vice-president at the end of the last administration,” Pence’s lawyer, Greg Jacob, wrote in a letter to the National Archives, shared with The Associated Press.

He said that Pence had been “unaware of the existence of sensitive or classified documents at his personal residence” until a search last week and “understands the high importance of protecting sensitive and classified information and stands ready and willing to cooperate fully with the National Archives and any appropriate inquiry”.

The revelation came as the Department of Justice was already investigating the discovery of documents with classification markings in President Joe Biden’s home in Delaware and his former Washington office, as well as former President Donald Trump’s Florida estate. Democrat Biden has indicated he will seek re-election, Trump is already a declared candidate, and Republican Pence has been exploring a possible 2024 presidential campaign that would put him direct competition against his former boss.

The newest discovery, which was first reported by CNN, thrusts Pence, who had previously insisted that he followed stringent protocols regarding classified documents, into the debate over the handling of secret materials by officials who have served in the highest ranks of government.

Trump is currently under criminal investigation after roughly 300 documents with classified markings, including at the top-secret level, were discovered at Mar-a-Lago. Officials are trying to determine whether Trump or anyone else should be charged with illegal possession of those records or with trying to obstruct the months-long criminal investigation. Biden is also subject to a special counsel investigation after classified documents from his time as a senator and in the Obama Administration were found at his properties.

Trump, who denies any wrongdoing, reacted to the new development on his social media site: “Mike Pence is an innocent man. He never did anything knowingly dishonest in his life. Leave him alone!!!”

While a very different case, the Pence development could bolster the arguments of Trump and Biden, who have sought to downplay the significance of the discoveries. The presence of secret documents at all three men’s homes further underscores the federal government’s unwieldy system for storing and protecting the millions of classified documents it produces every year.

Pence’s lawyer, Jacob, said in his letter that the former vice-president had “engaged outside counsel, with experience in handling classified documents” to review records stored at his home on January 16 “out of an abundance of caution” amid the uproar over the discovery of documents at Biden’s home.

Jacob said the Pence documents with classification markings were immediately secured in a locked safe. According to a follow-up letter from the lawyer, dated January 22, FBI agents visited Pence’s residence the night of January 19 at 9:30 pm to collect the documents that had been secured. Pence was in Washington for an event at the time.

A total of four boxes containing copies of administration papers — two in which “a small number” of papers bearing classified markings were found, and two containing “courtesy copies of vice-presidential papers” — were discovered, according to the letter. Arrangements were made to deliver those boxes to the National Archives Monday.

The National Archives did not immediately respond to a request for comment on the discovery. A Justice Department spokesman declined to comment Tuesday, and a lawyer for Pence did not immediately respond to an e-mail seeking elaboration.

Pence told The Associated Press in August that he did not take any classified information with him when he left office.

Asked directly if he had retained any such information, he said, “No, not to my knowledge.”

In an interview this month with Fox Business, Pence described a “very formal process” used by his office to handle classified information as well as the steps taken by his lawyers to ensure none was taken with him.

“Before we left the White House, the attorneys on my staff went through all the documents at both the White House and our offices there and at the vice-president’s residence to ensure that any documents that needed to be turned over to the National Archives, including classified documents, were turned over. So we went through a very careful process in that regard,” Pence said.

On Capitol Hill, members of the Senate intelligence committee expressed incredulity over the mishandling of documents by top US officials.

Republican Senator John Cornyn of Texas noted that classified documents are only moved out of the committee’s offices in locked bags. “In my book, it’s never permissible to take classified documents outside of a secure facility or by some secure means of transport in between those secure facilities,” he said.

House Intelligence Chairman Mike Turner, a Republican, said he planned to request a formal intelligence review and damage assessment.

“No one is above the law,” added Republican Senator Rick Scott, another potential 2024 candidate. “I don’t know how anybody ends up with classified documents…I mean, every classified document I’ve ever seen has a big ‘Classified’ on it.”

Meanwhile, some Republicans pressed for a search of former President Barack Obama’s personal records.

An Obama spokesperson referred to a 2022 statement from the National Archives that said the agency took control of all of his records after he left office and “is not aware of any missing boxes of presidential records from the Obama Administration”.

Representatives of former presidents George W Bush and Bill Clinton and former Vice -President Dick Cheney said all of their classified records had been turned over to NARA upon leaving the White House.