In a letter to the U.S. Senate Judiciary Committee on Monday, an attorney for Republican donor Harlan Crow challenged the panel’s authority to investigate his friendship with Justice Clarence Thomas, which has come under intense scrutiny amid reports of years of undisclosed benefits showered on the justice.

“After careful consideration, we do not believe the Committee has the authority to investigate Mr. Crow’s personal friendship with Justice Clarence Thomas,” read the letter obtained by The National Law Journal. The letter was signed by Crow’s attorney, Michael D. Bopp of Gibson, Dunn & Crutcher, who specializes in congressional investigations and other white-collar matters.