A onetime White House lawyer under President Donald Trump warned him late last year that Trump could face legal liability if he did not return government materials he had taken with him when he left office, three people familiar with the matter said.

The lawyer, Eric Herschmann, sought to impress upon Trump the seriousness of the issue and the potential for investigations and legal exposure if he did not return the documents, particularly any classified material, the people said.

The account of the conversation is the latest evidence that Trump had been informed of the legal perils of holding on to material that is now at the heart of a Justice Department criminal investigation into his handling of the documents and the possibility that he or his aides engaged in obstruction.

In January, not long after the discussion with Herschmann, Trump turned over to the National Archives 15 boxes of material he had taken with him from the White House. Those boxes contained 184 classified documents, the Justice Department has said.

But Trump continued to hold on to a considerable cache of other documents, including some with the highest security classification, until returning some under subpoena in June and having even more seized in a court-authorized search of his Mar-a-Lago residence and private club in Florida by FBI agents last month.

The precise date of the late 2021 meeting between Trump and Herschmann was unclear. It was also unclear what, if any, awareness Herschmann had of what was in the boxes when the subject was discussed.

But by then, the National Archives had told associates of Trump's that it was missing documents such as original copies of his presidential correspondence with North Korean dictator Kim Jong Un and the letter left for him by President Barack Obama. Archives officials said they had been told by then that there were roughly two dozen boxes of documents that had been in the White House residence and which qualified as presidential records, which had never been sent to the archives.

By the time of the meeting, Herschmann, a former prosecutor, was not working with or for Trump, from whom the National Archives had spent months trying to procure missing material.

Trump thanked Herschmann for the discussion but was noncommittal about his plans for returning the documents, the people familiar with the conversation said.

Herschmann, who defended Trump during his first impeachment trial but tried to stop several efforts by outside advisers aimed at keeping him in power after he lost the 2020 election, declined to comment. A spokesperson for Trump did not immediately respond to a request for comment.

The meeting between Herschmann and Trump has not been previously reported, and it adds to the picture of Trump's interactions with several people about returning the documents in the months before the National Archives retrieved 15 boxes of material in January. When they went through the boxes, officials at the archives discovered that they contained nearly 200 individual classified documents.

It was not immediately clear if the meeting was solely related to the discussion about the documents, or if it was about other issues.

Some of Trump's advisers, including informal ones such as Tom Fitton, of the conservative legal advocacy group Judicial Watch, have told the former president that he could hold on to the documents as personal records, according to people briefed on their discussions.

Trump is facing not just the investigation over potential mishandling of government records but a number of other inquiries, including a wide-ranging Justice Department investigation into what led to the Jan. 6, 2021, attack on the Capitol and a state investigation in Georgia into efforts to overturn the outcome of the 2020 election.

Lawyers for Trump turned over an additional set of classified documents in June. The FBI then carried out a search warrant at Mar-a-Lago on Aug. 8 and retrieved more than 100 additional individual documents with classified markings.

A federal judge in Florida has at least temporarily barred the Justice Department from using the material seized in the search in pursuing its criminal investigation. On Friday, the department asked a federal appeals court to let the FBI regain access to those 100 or so sensitive documents so it could continue the inquiry and assess the national security risks stemming from Trump keeping them in an unsecured location.

The special master appointed to determine whether the material seized in the search is subject to attorney-client privilege or executive privilege is scheduled to meet with lawyers for Trump and the Justice Department on Tuesday.

The first filings before Judge Raymond Dearie, the special master, suggested that the Trump legal team was not happy with early signs of how quickly he appears poised to try to resolve the matter.

Dearie had invited the Justice Department and the Trump legal team to submit letters Monday proposing what they should talk about at a first meeting in his courtroom in the federal courthouse in Brooklyn on Tuesday.

He also circulated a proposed calendar, which was not made public, for how the work flow could unfold.

In its submission, the Trump legal team complained about that calendar. For example, Dearie apparently proposed that both sides complete their sifting of the documents and proposals for how to label them by Oct. 7. After that deadline, Dearie would write a report to a Trump-appointed judge, Aileen Cannon, who named him special master and set an overall deadline of Nov. 30, recommending how she should rule about any disagreements.

The Trump legal team said Oct. 7 was far too early a date for that phase of the work to be done, writing: "We respectfully suggest that all of the deadlines can be extended to allow for a more realistic and complete assessment of the areas of disagreement."

Trump's team had recommended Dearie as a possible special master, and the Justice Department agreed.

The website Axios reported that the Trump team did so because the lawyers believed that the judge shared their skepticism of the FBI, because he was one of the judges on the Foreign Intelligence Surveillance Court who approved of warrants to surveil a 2016 Trump campaign adviser named Carter Page. Two of the four warrants the court approved were later declared invalid.