The IRS special agent-turned-whistleblower formerly known as "Mr. X." spoke out to Fox News on Friday following an at-times contentious congressional hearing earlier this week.

Joseph Ziegler, who came forward to Congress along with his colleague, Gary Shapley, said his team uncovered "a ton of evidence" that proved Hunter Biden allegedly willfully evaded or fraudulently filed his taxes, which set him apart from a typical IRS investigatory subject that would be faced with civil fines.

Ziegler also told "Special Report" the ultimate reason he came forward as a whistleblower was that he saw many instances where federal officials were not following proper procedures.

"I thought it was my duty to come forward," he said, going on to explain why he joined Shapley: "I thought when we were invited to come and testify in front of the congressional committee, I didn't want to leave my supervisor, Gary, there alone. And I thought it was important that for my credibility, that people hear it straight from me."

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Ziegler said he and Shapley were simply following the evidence without regard to politics or deference to or against the subject, Hunter Biden.

He added that in August 2022, all four prosecutors assigned to the case recommended felony and misdemeanor charges, adding that observers have cited that in many cases, IRS violations are referred for civil – not criminal – prosecution.

"[W]hen you work a criminal case, there's a thing called willfulness: whether someone intentionally with knowledge either evaded their income taxes or filed a false return. And that's what one thing that sets us apart from a civil case. And in this case, we provided a ton of evidence that showed willfulness," he said.

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He also commented on Assistant U.S. Attorney for the District of Delaware Lesley Wolf purportedly working to "limit" – in Shapley's words – questioning relating to President Biden and any references to "the big guy" in Hunter's correspondence.

Ziegler said Wolf, a deputy of lead Delaware prosecutor David Weiss, gave the investigation a "shot to the gut" when he and Shapley discussed a potential search warrant for a storage unit in Hunter Biden's name.

"When I found out that she told defense counsel that we knew about the storage unit and that the records request included that, I just knew – it was kind of like a shot to the gut in that we didn't have a seat at the table," he said.

"And that really was a defining moment for me that said things … aren't following the proper procedure."

"When you work an investigation, you have to follow the evidence. It's a matter of, if there is evidence laid out before you and there's potential tax crimes that exist, you should be able to follow the evidence."

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Ziegler also said his communications with the District of Columbia U.S. Attorney's office were initially met with overtures of cooperation, but that suddenly that stopped.

Line attorneys or career employees in the office, now led by Biden-appointed prosecutor Matt Graves, said they were going to help investigators expand their case to Washington.

"Then I get a phone call a couple of days later that says, 'Hey, they're not going to bring it. They don't want to partner with us. They're not going to help us. And they also say that, no, you shouldn't bring this case in our district'."

Ziegler said he and FBI investigators his team was working with were trying to figure out how to get an independent prosecutor with authority outside of Delaware's three counties – which Weiss oversees – assigned to their probe after being stymied in Washington.

He said he was ultimately removed from the team investigating Hunter Biden on May 15, and that he has no further window into the investigation, but added he does not want to see the current agents at the FBI and IRS deal with the same he has.

"If there are other charges that are viewed outside the District of Delaware. David Weiss is going to run into the same problem again and again. And that's why we need to have an independent attorney assigned who has authority, who can go in there, bring the proper charges, so that there can be some faith restored in our justice system."