House Republicans issued a subpoena Tuesday to a federal prosecutor involved in the criminal investigation into Hunter Biden, demanding answers for what they allege is Justice Department interference in the yearslong case into the president’s son.

Rep. Jim Jordan, chair of the House Judiciary Committee, called on Lesley Wolf, the assistant U.S. attorney for Delaware, to appear before the committee by Dec. 7, according to a copy of the congressional subpoena obtained by The Associated Press.

“Based on the Committee’s investigation to date, it is clear that you possess specialized and unique information that is unavailable to the Committee through other sources and without which the Committee’s inquiry would be incomplete,” Jordan wrote in an accompanying letter to Wolf.

The Justice Department did not immediately respond to a request for comment.

The subpoena to Wolf is the latest in a series of demands Jordan and fellow Republican chairmen have made as part of their sprawling impeachment inquiry into President Joe Biden. His youngest son Hunter and brother James received subpoenas last week as Republicans look to gain ground in their nearly yearlong investigation, which has so failed to uncover evidence directly implicating the president in any wrongdoing.

The inquiry is focused both on the Biden family’s international business affairs and the Justice Department’s investigation into Hunter Biden, which Republicans claim has been slow-walked and stonewalled since the case was opened in 2018.

Wolf, who serves with David Weiss, the U.S. attorney for Delaware in charge of the case, has been accused by whistleblowers from the Internal Revenue Service of “deviating from standard investigative protocol” and showing preferential treatment because Hunter Biden is the president’s son.

Republicans have claimed that it was clear that the prosecutors didn’t want to touch anything that would include Hunter Biden’s father. In one instance, Gary Shapley, an IRS employee assigned to the case, testified that in a meeting with Weiss and Wolf after the 2020 election, he and other agents wanted to discuss an email between Hunter Biden associates where one person made reference to the “big guy.” Shapley said Wolf refused to do so, saying she did not want to ask questions about “dad.”

Other claims relate to an August 2020 email in which Wolf ordered investigators to remove any mention of “Political Figure 1,” who was known to be Biden, from a search warrant. In another incident, FBI officials notified Hunter Biden’s Secret Service detail in advance of an effort to interview him and several of his business associates in order to avoid a potential shoot-out between two law enforcement bodies.

Justice Department officials have countered these claims by pointing to the extraordinary set of circumstances surrounding a criminal case into a subject who at the time was the son of a leading presidential candidate. Department policy has long warned prosecutors to take care in charging cases with potential political overtones around the time of an election, to avoid any possible influence on the outcome.