The Department of Justice improperly redacted a court filing related to the Mueller investigation and must reveal the names of two individuals who figured prominently in the probe, a federal judge in Washington ruled on Thursday.

Judge Beryl Howell of the U.S. District Court for the District of Columbia said in her opinion that the department erroneously redacted a portion of a document after invoking grand jury secrecy protections, even though the two names that were concealed belonged to individuals who did not testify before a grand jury during the Mueller probe.

"DOJ's assertion that identifying individuals who did not testify before the grand jury as part of the Mueller investigation would reveal 'a matter occurring before the grand jury' is without merit and rejected," Howell wrote.

According to the judge, who was appointed by former President Obama, both of the people whose names were redacted "figured in key events examined in the Mueller Report."

Howell said that it does not appear that the same redaction error was made in the report that former Special Counsel Robert Mueller gave to Congress earlier this year.

The redactions in question came in an affidavit submitted to the court by Associate Deputy Attorney General Bradley Weinsheimer, who reviewed Mueller's report for classified information before it was transmitted to Congress.

Thursday's ruling stems from the House Judiciary Committee's effort to obtain grand jury materials and other information that was not included or redacted from the Mueller report.