A lawyer for former US Vice President Mike Pence found about 12 classified documents at Pence's home in Indiana last week, and they were transferred to the FBI, without it being clear what the documents are, their level of sensitivity, or their classification.

Sources told CNN that Pence's attorney, Greg Jacob, discovered the confidential documents at Pence's new home in Indiana, following the disclosure of classified materials discovered in President Joe Biden's private office and residence.

The discovery comes after Pence has said repeatedly that he does not have any classified documents.

Pence's lawyer said in a letter dated January 18 to the National Archives - seen by Reuters - that "out of an abundance of caution" Pence hired him to review records stored at his home after reports of documents found at Biden's home.

The lawyer began examining four boxes stored in the house where he found a small number of classified documents, then the vice president immediately secured those documents in a locked safe and the National Archives was notified, and in turn, the archive notified the Ministry of Justice, according to the lawyer.

In a separate letter, dated January 22, Jacob said the Justice Department had "exceeded standard procedures and requested direct possession" of the documents at Pence's residence.

With the vice president's approval, the attorney added, FBI agents came to Pence's home in Indiana at 9:30 p.m. local time on January 19 and collected the documents held in the safe.

In a letter to the National Archives obtained by CNN, Pence's lawyer wrote that "a small number of documents marked confidential" were inadvertently wrapped and taken to the vice president's home.

He added that Pence "was not aware of the presence of sensitive or classified documents in his personal home. The Vice President understands the paramount importance of protecting sensitive and classified information and stands ready and willing to cooperate fully with the National Archives and any appropriate enquiry."

Pence's lawyer said Pence's office in Washington, D.C., was also searched, and no classified materials or other records covered by the Presidential Records Act were discovered.

In turn, the Republican member of the US House of Representatives, James Comer, confirmed today, Tuesday, that classified official documents were found in Pence's house.

"Former Vice President Mike Pence notified us today of the discovery of classified documents at his home in Indiana. He agrees to fully cooperate with our investigation," Kummer, who heads the House Committee charged with overseeing the work of the executive branch, said in a statement.

Comer is overseeing the parliamentary investigation opened by the House of Representatives in the wake of similar documents being found at the home of Democratic President Joe Biden.

During the presidential transition, records from each administration are supposed to be turned over to the legal custody of the US National Archives, and it is illegal to intentionally or willfully remove or retain classified material.

The news about Pence comes as advisers specializing in the handling of classified documents by Biden and former President Donald Trump are investigating.

The revelations also come amid speculation that Pence is preparing to run for the Republican nomination for president in 2024.