Federal prosecutors investigating former President Donald Trump’s handling of classified material have a recording of Trump from 2021 discussing a sensitive military document he had kept after leaving the White House, two people briefed on the matter said.

In the recording, Trump suggested he knew the document was secret and had not declassified it, one person briefed on the matter said.

The existence of the recording could undermine Trump’s repeated claim that he had already declassified material that remained in his possession after he left office. Prosecutors are scrutinizing whether Trump obstructed efforts by federal officials to retrieve documents he took with him after leaving office and whether he violated laws governing the handling of classified material.

The recording was made during a meeting Trump held in July 2021 with people helping his former chief of staff, Mark Meadows, write a memoir, according to the people briefed on the matter.

Meadows did not attend the meeting, but at least two of Trump’s aides did. One, Margo Martin, routinely recorded the interviews he gave for books being written about him that year.

On the recording, Trump began railing about his handpicked chair of the Joint Chiefs of Staff, Gen. Mark Milley, who was described in media accounts at the time as having guarded against Trump’s striking Iran in the final days of the presidency, according to the people briefed on the matter.

Trump then began referencing a document that he had with him, saying that it had been compiled by Milley and was related to attacking Iran, the people briefed on the matter said. Among other comments, he mentioned his classification abilities during the discussion, one person briefed on the matter said.

The Justice Department obtained the recording in recent months, a potentially key piece in a mountain of evidence that prosecutors have amassed under special counsel Jack Smith since he was appointed in November to oversee the federal investigations into Trump.

In an interview with CNN on Wednesday night, James Trusty, a lawyer representing Trump in the case, indicated that the former president was taking the position that he had declassified the material he took with him upon leaving office.

“When he left for Mar-aLago with boxes of documents that other people packed for him that he brought, he was the commander in chief,” Trusty said. “There is no doubt that he [had] the constitutional authority as commander in chief to declassify.”