Prosecutors would face substantial hurdles in potentially pursuing charges against Ms. Carroll, who twice won cases against Donald Trump, or the billionaire who helped pay her lawyers.
The ruling was a blow to both President Trump, who had voluntarily dismissed the suit last week, and to the Justice Department, which used the suit to establish a fund likely intended for Trump allies.
In a stern order on Friday, a judge said that she wanted to investigate the circumstances surrounding President Trump’s efforts to settle the lawsuit in a way that benefited him and his allies.
The ruling allowed the Trump administration to continue to pursue changes that include enlisting the Postal Service in checking voters against a national database.
Ms. Carroll, who prevailed in a civil trial after accusing President Trump of sexual abuse, is the latest target in a Justice Department campaign going after his perceived enemies.
The surprising demand to appear at a hearing suggested new concern about the Trump administration’s efforts to repay the full $166 billion owed from illegally imposed tariffs.
The actor Matthew Perry was found unresponsive in a hot tub at his home in Los Angeles on Oct. 28, 2023. His assistant had injected him with at least three shots of ketamine that day.
Republican leaders in the state have asked the justices to clear the way for a congressional map that a lower court found discriminated against Black voters.
The men were arrested in 2024, accused of surveilling dissidents from Hong Kong in Britain. The case sent a chill through pro-democracy protesters who have fled the territory.