U.S. prosecutors in the criminal case against Ghislaine Maxwell asked a judge to keep some documents secret to protect an ongoing grand jury investigation.

“The grand jury investigation is active and ongoing,” and resulted in charges filed in July, the prosecutors said in a letter to U.S. District Judge Alison Nathan Friday. They didn’t provide any details of the investigation, but offered to provide them for the judge to review privately.

Maxwell is a former friend of Jeffrey Epstein, who killed himself last year in a Manhattan jail cell while facing charges he trafficked underage girls for sex. Maxwell is charged with helping Epstein sexually abuse girls.