Classified documents have been found in the home of former Vice President Mike Pence and turned over to the FBI for review, multiple sources familiar with the matter tell ABC News.
A lawyer for Pence conducted the search of Pence's home in Indiana last week and found around a dozen documents marked as classified, sources said. The search was done proactively and in the wake of the news that classified documents from before he was president were found in Joe Biden's home and old office at the Penn Biden Center, a Washington, D.C., think tank.
The Pence documents are undergoing a review by the Department of Justice's National Security Division and the FBI, the sources said.
The revelation makes Pence the third high-profile official to have classified material discovered at their residence in recent months, after Biden and former President Donald Trump, both of whom are now being investigated by special counsels under the Department of Justice.
Biden's attorneys have stressed that they quickly sought to return the classified records after they were found, beginning in November, though the president has been criticized for not publicly disclosing the matter until earlier this month.
Former President Donald Trump's Mar-a-Lago estate was searched by federal agents in August after what the federal government said was a months-long effort to retrieve documents that Trump resisted handing over. Trump has denied wrongdoing and asserted, without evidence, that he declassified the documents.
CNN first reported the discovery of classified materials at Pence's home.
In a letter sent last week to the National Archives, and obtained by ABC News, a representative for Pence wrote that Pence had engaged outside counsel on Jan. 16 to review records that were stored in his home. It was during that review that a lawyer found a "small number of documents that could potentially contain sensitive or classified information interspersed throughout the records."
Pence's lawyer and representative, Greg Jacob, wrote in the letter that the counsel was unable to provide an exact description of the folders or briefing materials because they did not review the contents after realizing they had potential classification markings.
The materials seemed to have been "inadvertently boxed and transported" to Pence's home, Jacob wrote, and he had been "unaware" of their existence.
"Vice President Pence immediately secured those documents in a locked safe pending further direction on proper handling from the National Archives," Jacob, who is Pence's designated representative for his records and also his former top lawyer during the administration, wrote in the letter.
Jacob wrote that Pence was "willing to cooperate fully with the National Archives and any appropriate inquiry."
In a second letter to the National Archives sent on Sunday, Jacob wrote that the Department of Justice requested direct possession of the documents and that Pence agreed to them taking possession, even though he was in Washington, D.C., at the time.
Jacob wrote that FBI agents came to Pence's Indiana home last Thursday night to collect the documents. The transfer of the documents was facilitated by Pence's personal lawyer, who conducted the prior review on Jan. 16.
In his letter, Jacob disclosed that there were four boxes containing administration papers, which included two boxes with papers with classified markings as well as "two separate boxes containing courtesy copies" of Pence's vice presidential papers.
The boxes containing the documents bearing classification markings were first transported to Pence's temporary home in Virginia prior to being transported to the Indiana residence, sources familiar with the matter said. The boxes were taped, according to the sources, who said they did not appear to have been opened since they were packed.
A lawyer for Pence, Matt Morgan, searched the four boxes last week. After material containing classified markings was separated, placed into a safe and retrieved by the FBI, the four boxes were driven by Pence's legal team to the National Archives on Monday so the agency could review the materials in accordance with the Presidential Records Act.
Pence aides also searched the office of Pence's advocacy group in Washington, D.C. -- Advancing American Freedom -- and found no government documents or documents with classification markings.
A spokesperson for Pence declined to comment on whether the former vice president would invite the FBI to search his home, similar to what Biden did.
Pence previously told ABC News' David Muir that he did not retain any classified information after leaving office.
"Let me ask you, as we sit here in your home office in Indiana, did you take any classified documents with you from the White House?" Muir asked in a November interview.
"I did not," Pence said then. Asked if he saw "any reason for anyone to take classified documents with them, leaving the White House," he said, "There'd be no reason to have classified documents, particularly if they were in an unprotected area."
Attorney General Merrick Garland has appointed a special counsel to determine if any laws were broken in Biden and Trump's handling of classified materials while out of office.
On Tuesday, Garland declined to comment on the documents found at Pence's home.
Separately, House Oversight Committee Chairman James Comer, R-Ky., said in a statement on Tuesday that Pence "reached out today about classified documents at his home ... He has agreed to fully cooperate with congressional oversight and any questions we have."
Comer contrasted that "transparency" with what he said was the Biden administration's reluctance to provide more information about his handling of classified records.
Biden's attorneys have said his documents were "inadvertently misplaced" and emphasized cooperation with the Department of Justice. They've said they immediately turned over the materials to the appropriate authorities and consented to an FBI search of his Wilmington, Delaware, home on Friday.
The first set of Biden documents were found on Nov. 2 -- just a week before the midterm elections.
The White House on Tuesday declined to comment on the records at Pence's home.
Lawmakers on the Senate Intelligence Committee reacted more colorfully.
"I kind of thought 'holy heck,'" Chairman Mark Warner, D-Va., told reporters when asked how he first reacted to news.
"What the hell is going on around here is my reaction," Florida's Marco Rubio, the ranking Republican member said.
Both men stressed that they want more information before they can assess the situation and said a briefing from the intelligence community was key to understanding the classified documents issues.
Rubio and Warner agreed there may be "systemic problems" in how the executive branch deals with classified documents and over-classifications.
John Cohen, an ABC News contributor and former Department of Homeland Security undersecretary, said that in many cases it's an "accident" that such material is taken out when an official leaves office.
"I suspect that because of the visibility that the Biden case and the Trump case have generated, that you have a large number of former government officials -- whether they be former presidents or others -- who are looking in their basement at boxes that they've stored there since the last government," Cohen told ABC's "Start Here" podcast.
But Cohen acknowledged then the process for handling the bulk of classified information is problematic and lacks a tracking system.
Only a small subset of classified material is required to undergo a sign-in and sign-out process, Cohen explained.
"The government over the last five, six, seven years has made strides in identifying behavior that could be suspicious on these government systems ... But this idea that every classified paper document is being tracked? That's just not how the system works," he said.
ABC News' Justin Gomez, Alexandra Hutzler, Allison Pecorin and Will Steakin contributed to this report.