Mr. Welters, in response to questions from the Finance Committee, shared loan documents dated Dec. 6, 1999, showing that he lent Justice and Mrs. Thomas the entire purchase amount at an annual interest rate of 7.5 percent. While that rate was in line with what might have been found in the marketplace at the time, what made the arrangement unusually favorable was that over the course of the five-year loan, Justice Thomas did not have pay down any of the principal.

Instead, he simply had to make annual interest payments of $20,042. The principal amount borrowed would come due in a balloon payment on the loan’s maturity date, in December 2004. Vehicle loans like this one are very uncommon, experts said, because of the risk to the lender: The value of the collateral securing the loan — in this case, a motor coach — depreciates rapidly, while the outstanding principal remains constant.

In a handwritten note to Mr. Welters on his Supreme Court letterhead, dated the same day the loan documents were signed, Justice Thomas said the loan agreement should accurately reflect their understanding, and promised to abide by it to the letter, according to the Senate report.

But in 2004, when the principal came due, Justice Thomas did not make good on his debt, according to records obtained by the committee and cited in their report. Instead, Mr. Welters granted him a 10-year extension, with the same interest-only terms. This, despite the fact that the previous year Justice Thomas had collected $500,000 of a $1.5 million advance for his autobiography, according to his financial disclosures.

Then, in late 2008, Mr. Welters simply forgave the balance of the loan, according to the committee’s report.