Justice Department has ramped up its efforts in its probe of Donald Trump's Mar-a-Lago documents, looking to determine whether the former US president purposely hid the classified materials from his presidency.

The New York Times reported that there is an insider witness, whose identity has not been disclosed.

A picture of the storage room where the classified materials were held had been given to investigators by a witness.

It was not immediately known what the prosecutors learned from the witness or when the witness first started providing information to the prosecutors.

At least four more Mar-a-Lago employees have been subpoenaed in the past few weeks, including another person who had visibility into Trump's thinking when he first returned material to the National archives.

Prosecutors also subpoenaed Matthew Calamari Sr. Calamari was a previous Trump Organization head security, who became its chief operating officer.

The son of Calamari Sr., Matthew Calamari Jr., was also subpoenaed a long time ago, according to a person with knowledge of the matter.

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Mar-a-Lago Documents Probe

The investigation over the classified materials found at Trump's residence was led by Special Counsel Jack Smith.

Investigators are now looking into whether Donald Trump failed or refused to follow the request on the documents' return after his presidential term.

Returning such documents are required under the Presidential Records Act, as reported by The Hill.

Smith leads two inquiries regarding Donald Trump. The second one focuses on alleged efforts of the former president and member of his inner circles to hinder the tranfer of power in 2020 election, which he lost to US President Joe Biden.

Smith's team has also previously questioned some Trump employees about the surveillance footage, asking if it could have been tampered with, as cited by New York Post.

Walt Nauta, an aide to the former president, was also on the list of interest of prosecutors, particularly his text message to Calamari Sr., and conversations about the surveillance footage.

Nauta said that he was instructed by Trump to move the document-filled boxes out of the Mar-a-Lago storage closet, based on reports of what he told the Federal Bureau of Investigation.

Donald Trump Taking Mar-a-Lago Documents

The Justice Department wants to know how the classified records were found at the former president's residence.

The former president's legal team said that the sent the box and the laptop with the sensitive materials to the special counsel.

People familiar with the matter noted that a Trump aide took the box to Mar-a-Lago's "tennis cottage," where she worked.

No scanning machine was available at the time, deciding to turn the documents into scanned files using an Adobe application on her phone, according to CNN.

The people familiar with the matter said that while the aide was working on the pages, she did not notice that there were classified documents including the presidential records.

Trump lawyer James Trusty said in an NBC News report in April that there are no more classified documents at Trump's Mar-a-Lago estate.

Trusty is a former Justice Department colleague of special counsel Jack Smith.

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Written by: Mary Webber

WATCH: Classified documents found at Mar-a-Lago - from KTLA 5