Democratic lawmakers are calling for an investigation of Supreme Court Justice Clarence Thomas after ProPublica revealed Thursday that he did not report real estate deals made with Harlan Crow, a Dallas business executive and influential Republican donor to causes related to the law and judiciary.

According to ProPublica, Crow purchased three properties in Savannah, Ga., from Thomas in 2014, including the single-story house where Thomas's mother was living and two vacant lots nearby. Thomas did not disclose the $133,363 real estate transaction on his financial disclosure forms, as federal law would have required. Crow told the outlet that he wanted to preserve the first property as a museum dedicated to Thomas in the future, and said he had spent tens of thousands of dollars on improvements to the house "to preserve its long-term viability."

Thomas has not publicly commented on the latest ProPublica report, and neither Thomas nor representatives for the Supreme Court responded to requests for comment Thursday.

The report came on the heels of another ProPublica investigation that revealed that Thomas had accepted trips around the globe from Crow for more than two decades, including travel on a superyacht and private jet, without disclosing them. Thomas said last week that he had been advised "by colleagues and others in the judiciary" that those luxury trips counted as "personal hospitality" that did not have to be disclosed.

The first report prompted Senate Democrats to publicly urge Chief Justice John G. Roberts Jr. to start his own investigation into gifts to Thomas from Crow, and reinvigorated calls for the Supreme Court to adopt a binding code of ethics. The revelation of Thomas's additional, unreported real estate deals with Crow further outraged Democrats and led to new calls for Thomas to step down.

Sen. Sheldon Whitehouse, D-R.I., and Rep. Hank Johnson, D-Ga., called on the Judicial Conference, the federal court system's policymaking body, to refer Thomas to the U.S. attorney general for potential ethics violations. In addition to the justice's unreported gifts and real estate deals, the lawmakers noted that Thomas admitted in 2011 he had failed to report $680,000 of his wife's income from a conservative think tank.

"Given this history, there is at least reasonable cause to believe that Justice Thomas intentionally disregarded the disclosure requirement to report the sale of his interest in the Savannah properties in an attempt to hide the extent of his financial relationship with Crow," wrote Whitehouse and Johnson, members of the Senate and House judiciary committees, respectively.

In a statement, Whitehouse scoffed at the idea that Crow would have bought Thomas's properties "and deck them out for your family members to continue living in" as simply a friend.

"And if all of this was on the up and up, why didn't Justice Thomas disclose it to the American people as the law clearly requires?" Whitehouse said. "The Supreme Court justices are so deeply ensconced in a cocoon of special interest money that they can no longer be trusted to police themselves without proper process."

Rep. Don Beyer, D-Va., called on Thomas to resign, after Rep. Alexandria Ocasio-Cortez, D-N.Y., said last week that the House should impeach the justice. Rep. Jamaal Bowman (D-N.Y.) said Thomas "must resign or be impeached." Other lawmakers vented their frustration at a lack of accountability.

"For a society to hold together, there have to be SOME shared norms," Sen. Chris Murphy, D-Conn., tweeted Thursday night. "If we can't agree that it's wrong for a Supreme Court justice to secretly take free trips and gifts and money from a billionaire who regularly funds efforts to influence the court, then, man, I just don't know."

On Friday, the watchdog group Citizens for Responsibility and Ethics in Washington (CREW) filed a civil and criminal complaint against Thomas, saying that his acceptance and failure to disclose "repeated, lavish gifts" undermined confidence in the Supreme Court as an institution.

"Justice Thomas has accepted consistent and luxurious gifts to such an extent that they have subsidized his lifestyle in a way unimaginable for most judges, suggesting a use of his position for personal gain and a compromise of his objectivity," CREW President Noah Bookbinder said in a statement.

Thomas has confirmed he befriended Crow after he joined the Supreme Court in 1991, and said Crow and his wife are among his "dearest friends."

Thomas has reported receiving only two gifts since 2004, the year the Los Angeles Times revealed that he had received far more lavish gifts compared to his fellow justices. Federal law mandates that top officials from the three branches of government, including the Supreme Court, file annual forms detailing their finances, outside income and spouses' sources of income, with each branch determining its own reporting standards.

Judges are prohibited from accepting gifts from anyone with business before the court. Until recently, however, the judicial branch had not clearly defined an exemption for gifts considered "personal hospitality." Last month, a committee of the Judicial Conference revised those rules to be more specific.

In his statement last week, Thomas noted this "new guidance."

"And, it is, of course, my intent to follow this guidance in the future," he said.

Thomas has long been under scrutiny for possible conflicts of interest. Last year, Senate Democrats called on Thomas to recuse himself from certain cases after it was revealed that his wife, Virginia "Ginni" Thomas, had pressed the Trump White House in text messages to try to overturn the results of the 2020 presidential election.

Thomas was the only justice to dissent in the Supreme Court's decision in January to reject Donald Trump's request to block documents from being released to the House select committee investigating the Jan. 6, 2021, insurrection.

The Washington Post's Robert Barnes and Ann E. Marimow contributed to this report.