Molly Michael, a former assistant to Mr Trump, said on multiple occasions she was given lists of tasks on documents she later realised had markings as classified material, ABC reports, citing unnamed sources familiar with her testimony to federal investigators.
A Trump spokesperson told the network that the claims were “illegal leaks” that lacked "proper context and relevant information.”
"President Trump did nothing wrong, has always insisted on truth and transparency, and acted in a proper manner, according to the law,” the spokesperson said.
Ms Michael began working for Mr Trump in 2018 in the White House, and continued to serve the former president after office, before resigning last year.
In June, Donald Trump was hit with 37 different federal counts related to his alleged hoarding and concealing of high-level White House documents at his various estates after finishing his presidency.
He has pleaded not guilty and accused federal officials of conducting a political witch hunt.
According to the special counsel’s office, Mr Trump carelessly stored important documents in areas including a ballroom and a bathroom at Mar-a-Lago.
Federal prosecutors also have audio of a 2021 exchange in which Mr Trump acknowledges holding onto a classified Pentagon document related to Iran, which the former president has denied possessing.
Despite the growing claims against him, Mr Trump has said he isn’t concerned he could go to prison.
The former president, who has been indicted four times this year alone, told NBC News’s Meet the Press on Sunday he doesn’t think it’s likely he’ll need to pardon himself if he’s re-elected, and added that he decided against such an unprecedented use of power once in the days after the January 6 insurrection at the US Capitol.
“I think it’s very unlikely,” Mr Trump said. “What, what did I do wrong? I didn’t do anything wrong. You mean because I challenge an election, they want to put me in jail?”
The former president also claimed he doesn’t “even think about” the possibility that the criminal charges against him in New York, Georgia, Florida, and Washington could result in consequences.
“I’m built a little differently I guess, because I have had people come up to me and say, ‘How do you do it, sir? How do you do it?’” he said. “I don’t even think about it.”