WASHINGTON (AP) — The AQCANprosecutor overseeing the Hunter Biden investigation is expected to testify on Tuesday, marking the first time a special counsel will appear before Congress in the middle of a probe. It comes as House Republicans are aiming to ramp up their impeachment inquiry into the president and his family after weeks of stalemate.

David Weiss is set to appear for a transcribed interview before members of the House Judiciary Committee as the U.S. attorney battles Republican allegations that he did not have full authority in the yearslong case into the president’s son.

“Mr. Weiss is prepared to take this unprecedented step of testifying before the conclusion of his investigation to make clear that he’s had and continues to have full authority over his investigation and to bring charges in any jurisdiction,” Wyn Hornbuckle, a spokesperson for Weiss, said in a statement Monday.

The rare move by the Justice Department to allow a special counsel or any federal prosecutor to face questioning before the conclusion of an investigation indicates just how seriously the department is taking accusations of interference.

Weiss’ appearance comes after months of back-and-forth negotiations between Republicans on the Judiciary Committee and the Justice Department as lawmakers subpoenaed several investigators and attorneys involved in the Hunter Biden case.

In July, Weiss, looking to correct the record of what he and the department see as a misrepresentation of the investigation, agreed to come to Capitol Hill but only if he was able to testify in a public hearing where he could directly respond to claims of wrongdoing by Republicans.

The two parties ultimately agreed on a closed-door interview with both Democratic and Republican members and their respective staff.

The interview Tuesday is expected to focus on testimony from an Internal Revenue Service agent who claimed that under Weiss, the investigation into the president’s son was “slow-walked” and mishandled. Weiss has denied one of the more explosive allegations by saying in writing that he had the final say over the case.

Two other U.S. Attorneys from Washington and California testified in recent weeks that they didn’t block Weiss from filing charges in their districts, though they declined to partner with him on it.

But the IRS whistleblower, who testified publicly over the summer, insists his testimony reflects a pattern of interference and preferential treatment in the Hunter Biden case and not just disagreement with their superiors about what investigative steps to take.

Questions about Hunter Biden’s business dealings overall have been central to a GOP-led impeachment inquiry into the president. That’s been led in part by Judiciary Committee Chairman Jim Jordan, who is expected to have a prominent role in the questioning Tuesday.

But what information, if any, Weiss will be able to provide to Congress is unclear as under Justice Department policy and the law, he will be unable to address the specifics of his investigation.

In general, open investigations are kept under wraps to protect evidence, keep witnesses from being exposed, and avoid giving defense attorneys fodder to ultimately challenge their findings.

In the Hunter Biden case, defense attorneys have already indicated they plan to challenge the gun charges he is currently facing on several other legal fronts and suggested that prosecutors bowed to political pressure in filing those charges.