Supreme Court Justice Clarence Thomas reportedly has been claiming thousands of dollars annually from a shuttered real estate firm

Supreme Court Justice Clarence Thomas reported on financial disclosure forms that his family has earned thousands of dollars in rental income from a Nebraska real estate firm that has been shuttered since 2006, according to a report by The Washington Post on Sunday.

Thomas has reported income from a firm called Ginger Ltd. Partnership over the last two decades, but in 2006 it was shut down and replaced by a new firm, the report said. The new firm, Ginger Holdings LLC, is similarly named, but there is no mention of it in Thomas' records.

In recent years, Thomas reportedly continued to disclose between $50,000 and $100,000 in income from the old firm annually.

Even if the misstatement can be reduced to a paperwork error, it marks the latest question around the justice's financial practices after a recent ProPublica report revealed Thomas has accepted secret luxury trips from Republican megadonor Harlan Crow for more than two decades in apparent violation of a financial disclosure law.

Thomas, the 74-year-old conservative associate justice who has served on the nation's highest court since 1991, has not reported the trips on his financial disclosures as required by law, the nonprofit newsroom reported. ProPublica later reported that Crow bought property from Thomas as well, which the justice also failed to disclose.

The investigation offers more fuel for Thomas' critics, who say his refusal to recuse himself from cases touching on issues related to his wife's political work in conservative circles — including her involvement in schemes to overturn the 2020 election — poses a conflict of interest.

The Senate Judiciary Committee's Democratic majority on Monday called for an investigation into Thomas' behavior. Chief Justice John Roberts should "immediately open" a probe into "how such conduct could take place" on his watch, read a letter from Chairman Dick Durbin of Illinois and the Senate Judiciary panel's 10 other Democratic members.

The committee announced in the letter that it would hold a hearing "in the coming days" on "the need to restore confidence in the Supreme Court's ethical standards."

Thomas and his wife did not respond to the Post's requests for comment.

— CNBC's Kevin Breuninger contributed to this report.