Lawyers from Jones Day on Friday apologized to a Virginia federal judge for exposing secret grand jury information in a court filing, an error the law firm attributed to a failure by the legal team to use certain software that is designed to "avoid such issues."

Jones Day attorneys are representing the drug company Indivior Inc., charged in the U.S. District Court for the Western District of Virginia with the alleged fraudulent marketing of a prescription opioid. The company, contesting the Justice Department's charges, recently filed court papers arguing the case should be dismissed for alleged government misconduct.

The original filing, a reply memorandum, exposed testimony that was protected by grand jury secrecy rules, and a U.S. magistrate judge on Wednesday ordered Jones Day to explain how that happened, why the firm should not face sanctions and what's being done to prevent any subsequent mishaps.

"We very much regret that this incident occurred and can assure the court that it will not happen again," Cleveland-based Jones Day partner James Wooley said in Friday's filing. Wooley said co-counsel at the firm Gentry Locke submitted the filing at issue but "the redaction process for that pleading was entirely the responsibility of Jones Day."

Wooley said a team of Jones Day lawyers reviewed the court filing before it was submitted to the court to make sure any grand jury testimony had been redacted.

The filing appeared "fully" redacted, Wooley said. But it turned out that it was not redacted at all. A news reporter, Wooley said, "defeated" the redaction by copying the black-out boxes and pasting the text into a new document.

"This technical weakness in the redaction process was caused by the method of redaction, which involved Microsoft Word and printing to Adobe Acrobat, rather than the redaction software our law firm has in place that is specifically designed to avoid such issues," Wooley wrote. "The failure to use this software was inadvertent oversight."

The senior lawyers on the Indivior team, Wooley said Friday, "will make certain that all future redactions in this matter are made with the proper software to avoid such issues in the future."

Federal prosecutors are resisting Indivior's push to dismiss the indictment for alleged government misconduct. The company's claims are centered on the government telling the grand jury about the prior criminal conviction of a third-party doctor.

"Tellingly, Indivior has not even attempted to argue that the indictment does not present a case against it, or that the other facts alleged in the indictment, as well as the plethora of evidence presented to the grand jury, was insufficient for the grand jury to find probable cause," prosecutors said in August.

U.S. Magistrate Judge Pamela Meade Sargent had not immediately responded Friday afternoon to Jones Day's apology. Sargent had given the firm two weeks—until the end of September—to explain how the redaction error occurred.

Jones Day's filing came within days of the judge's show-cause order.

Jones Day's notice to the court is posted below: