The attorney representing IRS whistleblower Gary Shapley said Thursday that President Biden appointed officials to "shut down" the possibility of prosecuting Hunter Biden for tax evasion, highlighting several instances the Department of Justice allegedly interfered to "curb" the probe into the first son.

Shapley, who served as the supervisor on the Biden investigation at the IRS and special agent Joseph Ziegler testified at the House Oversight Committee Wednesday about the "preferential treatment" they witnessed for the president's family and the "obstruction of the normal investigative process" that allegedly occurred throughout the years-long federal investigation into Hunter Biden. 

In an interview Thursday, Shapley's attorney Tristan Leavitt told Fox News' Martha MacCallum that prosecutors in Delaware were "on board" with bringing charges against the younger Biden for tax evasion based on evidence presented by IRS investigators. However, that option was "shut down" by Biden-appointed officials once the investigation picked up speed.

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IRS whistleblowers testify

IRS Supervisory Special Agent Gary Shapley, left, and Joseph Ziegler, an IRS Agent with the criminal investigations division, take their seats at a House Oversight and Accountability Committee hearing with IRS whistleblowers, Wednesday, July 19, 2023, in Washington. (AP Photo/Jacquelyn Martin) (AP)

"Prosecutors in Delaware were on board. They sent it to Justice Department's tax division and they recommended multiple felony charges for 2014 through 2019 as well as a number of misdemeanors. So it wasn’t just prosecutors versus investigators. Everybody was on board until the president appointed officials to shut the opportunities to prosecute down," he said on "The Story."

During a televised sit-down with Fox News' Bret Baier last month and in testimony before the House Ways and Means Committee, Shapley alleged that DOJ prosecutors directed investigators to avoid asking witnesses questions about President Biden and chose not to collect search warrants related to the president’s son. Shapley also identified U.S. Attorney Lesley Wolf as a key player who worked repeatedly to "limit" questioning related to Joe Biden.

Hunter Biden Air Force One

Hunter Biden disembarks from Air Force One at Hancock Field Air National Guard Base in Syracuse, New York, U.S., February 4, 2023. (REUTERS/Elizabeth Frantz)

Shapley said that Hunter Biden should have been charged with tax evasion for 2014, and false tax returns for 2018 and 2019 according to the evidence gathered by the IRS. With regard to the 2014 tax returns, Shapley said Hunter Biden did not report income from Ukrainian natural gas firm Burisma Holdings. 

"We weren’t allowed to ask questions about ‘dad.’ We weren’t allowed to ask about ‘the big guy.’ We weren’t allowed to include certain names in document requests and search warrants," Shapley said at the time. "So, you know, we were precluded from following that line of questioning." 

Leavitt said Wolf oversaw the first stage of the "interference" as investigators began gathering evidence into Hunter's tax history.

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"There’s really two different buckets of the interference. In terms of asking about that, these were decisions that the Delaware U.S. attorney’s office made and specifically U.S. attorney Leslie Wolf, she’s the one that repeatedly told them, you can’t ask about ‘the big guy’ or execute search warrants at the Biden residence or interview Hunter’s adult children, even though hunter was claiming tax deductions that involved those children," the attorney said. "So all of those were areas where the investigation itself was curbed."

IRS Whistleblowers at hearing

Internal Revenue Service whistleblowers Gary Shapley and Joseph Ziegler are sworn in so they may provide testimony in the House Oversight and Accountability Committee hearing about alleged meddling in the Justice Department's investigation of Hunter Biden, on Capitol Hill, Washington, U.S., July 19, 2023. (REUTERS/Leah Millis)

Eventually, prosecutors recommended charges be brought against Hunter, Leavitt said. David Weiss, the Trump-appointed U.S. attorney for Delaware who was leading the investigation, referred the case to U.S. Attorney for D.C. Matthew Graves and later to U.S. Attorney for the Central District of California Martin Estrada. Both Graves – who donated to Biden's 2020 campaign  - and Estrada "rejected the opportunity" to press charges against the president's son, Leavitt said.

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Weiss lacked the authority to bring charges own in Delaware and was "constantly hamstrung, limited, and marginalized" by DOJ officials as he sought to make prosecutorial decisions," according to testimony from Shapley. 

"So, Weiss had nowhere he could bring the cases because that’s how those are required to be charged," Leavitt told MacCallum.

The congressional probe, led by House Republicans, has centered around Shapley and Ziegler's claim there was a pattern of "slow-walking investigative steps" into Hunter Biden, which included instructions not to speak with him at his residence, tipping the president’s son and staff off about the ongoing efforts and delaying enforcement actions in the months before the 2020 presidential election.

Joe Biden waving with Hunter Biden

President Biden and his son Hunter Biden. (NICHOLAS KAMM/AFP via Getty Images)

In June, the Justice Department announced that Hunter Biden agreed to plead guilty to two misdemeanor counts of willful failure to pay federal income tax as part of what critics called a "sweetheart plea deal" that is expected to keep him out of prison. The president's son also agreed to enter into a pretrial diversion agreement with regard to a separate charge of possession of a firearm by a person who is an unlawful user of or addicted to a controlled substance. 

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The Justice Department has denied any improper interference in the investigation. 

Fox News' Brooke Singman contributed to this report.