The US Senate Judiciary Committee Tuesday sent a letter asking conservative billionaire Harlan Crow to reveal previously undisclosed gifts he and his companies made to US Supreme Court Justice Clarence Thomas.

Crow is a well-known Republican mega donor who built his fortune through real estate development. Specifically, the letter alleges Thomas failed to abide by the Supreme Court’s Statement on Ethics Principles and Practices, which went into effect last month, by not reporting gifts made by Crow. The committee also requested information from Crow’s companies that own private planes, yachts, and a campsite after allegations that Thomas accepted lodging, flights, and travel accommodations from them.

Previously in 2012, the Senate Judiciary Committee sent a letter to US Supreme Court Chief Justice John Roberts asking the Supreme Court to enact a code of conduct that would allow more accountability. In 1991, the justices adopted the Judicial Conference Regulations on gifts, which prohibit judicial officers or employees from accepting gifts under certain circumstances. For example, the regulations do not allow judges to accept gifts from people seeking to “do business with the court” or asking for any official action from a justice.

Thomas has until May 22 to comply with the committee’s request. Last week, the Washington Post reported Thomas’s wife accepted money for consulting work she completed for the president of the Federalist Society.