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.

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.

When questions were first raised last month, a court spokeswoman said 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, Thomas said the Federalist Society, a prominent conservative legal group, had reimbursed him an undisclosed sum for four days of “transportation, meals and accommodations” over the retreat weekend.

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,” Pearson said. He said if Thomas’ 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 Monday. Nor did officials at the Federalist Society or at Koch Industries.

Common Cause maintains that 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 Koches.

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

The Citizens United decision, with Thomas’ support, freed corporations to engage in direct political spending with little public disclosure.