Attorneys for Donald Trump turned over material in mid-March in response to a federal subpoena related to a classified US military document described by the former president on tape in 2021 but were unable to find the document itself, two sources tell CNN.

Prosecutors issued the subpoena shortly after asking a Trump aide before a federal grand jury about the audio recording of a July 2021 meeting at Trump's golf course in Bedminster, New Jersey. On the recording, Trump acknowledges he held onto a classified Pentagon document about a potential attack on Iran.

Prosecutors sought "any and all" documents and materials related to Mark Milley, Trump's chairman of the Joint Chiefs of Staff, and Iran, including maps or invasion plans, the sources say. A similar subpoena was sent to at least one other attendee of the meeting, another source tells CNN.

The sources say prosecutors made clear to Trump's attorneys after issuing the subpoena that they specifically wanted the Iran document he talked about on tape as well as any material referencing classified information -- like meeting notes, audio recordings or copies of the document -- that may still be Trump's possession.

The fact that Trump's team was unable to produce the document underscores the challenges the government has faced in trying to recover classified material that Trump took when he left the White House and in understanding the movement of government records that Trump kept.

Over the course of the Justice Department's investigation, prosecutors have expressed skepticism that all classified documents had been returned. The federal government recovered dozens of documents with classified markings from Trump at various points throughout 2022.

The special counsel's office complained late last year to a federal judge that they couldn't be sure Trump had turned over all documents with classified markings in his possession, even after the FBI search of Mar-a-Lago last August, CNN previously reported.

The dispute resulted in several sealed court proceedings where the prosecutors sought to hold Trump in contempt, but the judge declined at that time, and Trump's team hired two people to search his properties.

The Justice Department declined to comment.

Sources were unable to tell CNN if prosecutors already possess the Iran document and were seeking any copies still in Trump's possession -- or if the document is missing.

Trump's lawyers have said Trump and his staff did not review in advance materials in the boxes the former president returned to the National Archives last year and have not been told what documents were recovered in the subsequent FBI search of Trump's Mar-a-Lago resort in Florida.

Trump's lawyers wouldn't receive full access to classified documents seized in the FBI search and have only been able to view recently the boxes Trump returned to the Archives in January 2022 with markers in them in place of classified documents that had been returned in those boxes.

The document Trump references on tape was created before Milley was named as chairman of the Joint Chiefs of Staff, CNN was told, and Milley was later interviewed by investigators. Sources have been unable to specify if Trump had the document in question with him as he discussed it in the meeting in 2021 or was simply referencing it.

The recording, first reported by CNN, is now in the hands of special counsel Jack Smith, who is leading the Justice Department investigation into Trump. Smith has focused in recent months on the meeting as part of the yearlong criminal investigation into the former president's handling of national security secrets.

The audio of Trump acknowledging he had a classified document undercuts Trump's repeated claims that he declassified everything he took from the White House when he left office.

Trump on Thursday said he didn't know anything about the summer 2021 meeting and again called the Justice Department investigation a "witch hunt" and an attempt to interfere with the 2024 presidential election.

"I don't know anything about it," Trump said during a Fox News town hall. "I have the right to declassify as president."

Trump aides and two people working on the autobiography of Trump's former chief of staff Mark Meadows attended the Bedminster meeting with Trump. That autobiography includes an account of what appears to be the same meeting, where Trump recalls a report "typed up" by Milley and containing a plan to attack Iran.

The subpoenas were issued immediately after Trump aide Margo Martin, who attended the meeting, appeared before the grand jury in Smith's investigation and was asked about it. That's also when Trump's legal team found out prosecutors had the recording.

The Justice Department had access to a version of the recording before Martin's grand jury appearance in March, according to one source. Martin's attorney declined to comment.

After receiving the subpoena to Trump, his legal team spoke with aides and collected materials responsive to the subpoena, including the transcripts of recordings made by Trump aides and any other documents that mentioned Milley or Iran, sources say.

The legal team was unable to locate the document Trump mentions on the tape, the sources say, and it remains unclear if it was ever returned to the government or where it is now.

Trump attorney Jim Trusty declined to say on CNN this week whether the document was ever returned to the National Archives.

This story has been updated with additional details.