A federal grand jury is looking into how the former president handled classified documents. At least two dozen people, including Mar-a-Lago personnel and members of Donald Trump's inner circle, have been subpoenaed to appear in Donald Trump Mar-a-Lago case, according to sources familiar with the matter.

Trump's former White House employee and Florida resident, Margo Martin, testified before a grand jury in Washington, DC, on Thursday, according to CNN.

Jack Smith's top prosecutor was present for the interview as part of the special counsel.

Martin, one of a select few former White House aides who has remained on the Trump team after he left office, declined to comment when contacted by a CNN reporter.

Donald Trump's attorneys, representing him in the case, and Mar-a-Lago staff members, including a maid and servers, are among the persons Smith has reportedly asked to testify.

Investigators are interested in the employees because of the information these people may have learned while doing their jobs at the estate, such as whether they observed any boxes or documents in Trump's office suite or elsewhere.

"They're casting an extremely wide net – anyone and everyone who might have seen something," said one source familiar with the Justice Department's efforts.

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Donald Trump Mar-a-Lago Investigation Has Been Running for About a Year

A member of the Mar-a-Lago staff was seen on surveillance footage taking boxes out of a storage area, and it has been reported that they have spoken with investigators.

It has been nearly a year since the Justice Department began investigating the matter, The Daily Mail noted.

The investigation became public knowledge when FBI investigators raided Mar-a-Lago last August.

They discovered 11,179 government documents or images without any classification markings, along with 18 items identified as top secret, 54 as secret, 31 as confidential, and 18 as top secret.

The investigation into Donald Trump Mar-a-Lago case started when Trump's staff members were seen taking boxes to wait for helicopters as he left the White House; concerns were raised at the National Archives that presidential records had not been adequately turned over.

That prompted a check to see exactly what had been surrendered.

Another 15 boxes of records moved to Mar-a-Lago were returned to the archives a year later. Later, however, several were discovered to be classified.

The Former President Might Be Facing More Than Donald Trump Mar-a-Lago Case

Trump has denied wrongdoing on multiple occasions and claimed that the inquiry into sensitive information is just one of many "witch hunts" motivated by politics.

The former president asserts that he had a "standing order" that caused all records removed from the Oval Office to be automatically declassified. Still, he still needs to offer supporting documentation.

Meanwhile, Smith oversees two investigations into Trump: the Donald Trump classified documents scandal and another into Trump's role in the January 6 storming of the Capitol, but the former president may confront his most pressing legal concern in New York, Forbes reports.

The Manhattan district attorney's office reportedly urged Trump to testify in a case involving a hush money payment made to porn star Stormy Daniels late in the 2016 presidential campaign to prevent her from publicly disclosing an alleged romance with Trump, according to a New York Times report published last week.

Along with the criminal investigation in Georgia into Trump's efforts to change the results of the 2020 election, Trump is now the subject of seven New York criminal investigations for alleged financial fraud.

READ MORE: Donald Trump's Hush Money Payment to Porn Star Stormy Daniels 

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Written by: Bert Hoover

WATCH: Subpoenas issued to dozens of Mar-a-Lago staff - From 9NEWS