WASHINGTON — Discrepancies in reports about an appearance by Justice Clarence Thomas at a political retreat for wealthy conservatives three years ago have prompted new questions to the Supreme Court from a group that advocates changing campaign finance laws.

Discrepancies in reports about an appearance by Justice Clarence Thomas at a political retreat for wealthy conservatives three years ago have prompted new questions to the Supreme Court from a group that advocates changing campaign finance laws. When questions were first raised about the retreat last month, a court spokeswoman said Justice Thomas had made a “brief drop-by” at the event in Palm Springs, Calif., in January 2008 and had given a talk.

In his financial disclosure report for that year, however, Justice Thomas reported that the Federalist Society, a prominent conservative legal group, had reimbursed him an undisclosed amount for four days of “transportation, meals and accommodations” over the weekend of the retreat. The event is organized by Charles and David Koch, brothers who have used millions of dollars from the energy conglomerate they run in Wichita, Kan., to finance conservative causes.

Arn Pearson, a vice president at the advocacy group Common Cause, said the two statements appeared at odds. His group sent a letter to the Supreme Court on Monday asking for “further clarification” as to whether the justice spent four days at the retreat for the entire event or was there only briefly.

“I don’t think the explanation they’ve given is credible,” Mr. Pearson said in an interview. He said that if Justice Thomas’s visit was a “four-day, all-expenses paid trip in sunny Palm Springs,” it should have been reported as a gift under federal law.

The Supreme Court had no comment on the issue Monday. Nor did officials at the Federalist Society or at Koch Industries.

Common Cause maintains that Justice Thomas should have disqualified himself from last year’s landmark campaign finance ruling in the Citizens United case, partly because of his ties to the Koch brothers.

In a petition filed with the Justice Department last month, the advocacy group said past appearances at the Koch brothers’ retreat by Justice Thomas and Justice Antonin Scalia, along with the conservative political work of Justice Thomas’s wife, had created a possible perception of bias in hearing the case.

The Citizens United decision, with Justice Thomas’s support, freed corporations to engage in direct political spending with little public disclosure. The Koch brothers have been among the main beneficiaries, political analysts say.