A federal judge on Monday ordered prosecutors to turn over to defense lawyers additional information about a criminal investigation into an FBI agent in Washington accused of tampering with drug and gun evidence.

U.S. District Judge Emmet G. Sullivan said the lawyers would also get several internal FBI reports by Tuesday. But he put the documents under a protective order, meaning the defense lawyers are barred from revealing their contents to anyone but their clients.

The ruling came after Sullivan held a private meeting with prosecutors from the U.S. attorney’s offices for the District and the Eastern District of Pennsylvania. The latter office is leading the investigation of 33-year-old agent Matthew Lowry.

Sullivan, in brief remarks after the closed-door session, told the lawyers that they “will receive additional material” but did not describe it.

The judge’s order came in one of the many drug cases that Lowry’s alleged misconduct is affecting. Prosecutors in the District have notified 150 defendants that Lowry was involved in their cases. Of those, prosecutors have dismissed indictments against two dozen defendants, including some who had been convicted and sent to prison.

Still, details of the alleged evidence tampering have been shrouded in secrecy. The investigation began the week of Sept. 29, around the time officials have said the agent was found slumped in an unmarked FBI vehicle parked near the Navy Yard. Heroin and two guns were in the car, investigators said. Lowry, who has been suspended from the Washington field office, has not been charged with a crime. Authorities said Monday that the investigation would not take more than several months.

In most instances, prosecutors described Lowry’s involvement in drug investigations as peripheral and indicated they would not dismiss charges. But those cases have been put on hold until the Lowry investigation is complete, leading to delays in plea bargains, bail hearings and sentencings.

Monday’s hearing, which centered on one of the cases that has been put on hold, involved seven of the 30 defendants charged in a drug conspiracy indictment under the alleged kingpin, Juan Floyd. Prosecutors said Lowry participated in searches of a house and vehicle in Maryland. Court documents say the guns the agent allegedly tampered with were from that search.

Sullivan said he wanted to ensure that prosecutors in Pennsylvania — who are handling the investigation because D.C. officials have a conflict of interest — quickly disclose any information that might be favorable to defendants in Washington.

Before the hearing, prosecutors in the District said they planned to release additional details about Lowry.

Assistant U.S. Attorney Peter F. Schenck, the chief of the criminal division for the federal prosecutor’s office in Philadelphia, assured Sullivan that his office is cooperating. “What we know, they will know,” Schenck said. “When we know it, they will know it.” Schenck said no other agent or police official other than Lowry is a target of the investigation.

But defense lawyers said they want the names of all of the agents who worked the case and the ones who partnered with Lowry to determine whether they knew of any wrongdoing and failed to report it, which could be used to impeach their credibility. Before the judge’s ruling, defense lawyer Jonathan Zucker complained of what he called a “shutdown of information” and said the lawyers were “in the dark.”