Internal Revenue Service agents were left out of a briefing on an alleged "criminal bribery scheme" investigation into President Joe Biden and his family, Sen. Chuck Grassley (R-IA) claimed in a letter sent Sunday.

Grassley sent a letter to United States Attorney for the District of Delaware David Weiss, who led the investigation into Hunter Biden, claiming that IRS investigators were not "looped in" on a briefing involving an FD-1023 form that Republicans say proves members of the Biden family accepted money in exchange for policy from foreign groups.

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The Iowa senator said that senior prosecutor Assistant U.S. Attorney Lesley Wolf and FBI agents from Baltimore were briefed on the form in October 2020, but IRS investigators were not. Grassley said whistleblowers have affirmed that Wolf "prevented investigators from seeking information about Joe Biden’s involvement in Hunter Biden’s criminal business arrangements."

"In light of AUSA Wolf’s alleged questionable and obstructive conduct during the course of your investigation, I’m seeking clarification from you with respect to your knowledge of these allegations and how you’ve handled them," Grassley wrote.

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Both the House and Senate GOP have launched investigations into the Biden administration and Biden's family members since whistleblowers from the FBI, and IRS came forward claiming they were prevented or misled in their attempts to properly probe into the "alleged criminal scheme."

House Oversight Committee Chairman James Comer (R-KY) said last week he is "hopeful" that more of the House Oversight and Reform Committee's "disturbing findings" will be revealed this week. He believes the Biden family took in over $20 million from foreign sources.