The FBI agent accused of tampering with drug and gun evidence reportedly took heroin earmarked for trial and used it himself, according to two law enforcement officials familiar with an investigation that is prompting authorities to dismiss cases against convicted narcotics dealers in the District.

Robert C. Bonsib, the attorney for Matthew Lowry, the 33-year-old agent, declined to comment on specifics of the case, saying only that some accusations are “grossly overblown.” He added that Lowry, who has been suspended but not charged, wants to cooperate and “help bring this matter to a fast conclusion.” Law enforcement officials said the agent has talked to them and discussed the allegations.

Federal officials allege Lowry took drugs that were being stored in a lab for a future court case. Officials familiar with the case have also said that Lowry was found in late September slumped over the wheel of his unmarked FBI vehicle near the Navy Yard. Two additional officials said that in the car were two drug evidence bags, heroin and two firearms. Lowry is not named in the court documents.

Fallout from the investigation continued Thursday as a federal judge dismissed charges against 10 defendants in one large-scale drug case and vacated prison sentences that ranged from two to 12 years. The defendants had been released from prison Oct. 27, but with the judge’s order they became the first felons to be cleared of criminal charges in a case linked to the agent. In a separate case, another federal judge delayed dismissing charges against four suspects, saying he wanted additional information. A hearing is scheduled for Friday for six of 14 defendants in a third drug case.

In the cases that were dismissed Thursday, U.S. District Judge James E. Boasberg issued a two-paragraph order that does not describe the reasons. Typically in such cases, it is because unauthorized taking of evidence breaks what is called a “chain of custody,” and authorities no longer vouch for the integrity of the evidence. Boasberg set hearings for Nov. 14 and 17 during which additional details could be revealed.

The flurry of court hearings and filings this week underscored both the seriousness and the complex nature of separate law enforcement agencies simultaneously conducting a secretive investigation into the alleged misconduct while also trying to deal with its impact on criminal cases that involved the agent.

The U.S. Attorney’s Office in Washington has agreed to drop cases against 28 defendants, but various judges assigned to each need to approve.

Lowry worked out of the Washington field office on a cross-border task force, which involves local police and concentrates on crime along the District’s boundaries with Maryland and Virginia. Federal prosecutors in those states said they are reviewing cases to determine whether any involved Lowry. The U.S. Attorney’s Office in the Eastern District of Virginia said prosecutors had not found that the agent was involved in significant matters.

The FBI has said the investigation began the last week of September. No other details beyond the nature of the allegations have been publicly released, although court documents unsealed this week noted the accusations include alleged tampering with drug and gun evidence. Authorities have not commented further.

But on Thursday, Andrew Ames, spokesman for the FBI’s Washington field office, said that the “weapons referenced in the court filings are in the government’s possession.” He declined to elaborate but said the bureau wanted the public to know there are no missing guns on the streets.

Lowry’s attorney, Bonsib, declined to comment on specifics of the case, saying his client preferred to discuss the matter with authorities. “He hopes that by cooperating with the investigators, he can help them get a better idea of what happened and didn’t happen,” Bonsib said.

The lawyer said Lowry graduated with honors from the academy and “has had a distinguished career with the FBI. Law enforcement has been his life’s dream.” Bonsib said Lowry is “devastated by what has occurred here.” He repeatedly declined to describe the allegations being made against Lowry, saying only that “he intends to make himself fully available and help authorities get a handle on the real facts that occurred here.”

Lowry’s father, William Lowry, is the assistant chief of the Anne Arundel County police, had served 27 years on the Prince George’s County police force and headed security details for two NFL teams, including the Washington Redskins. The elder Lowry declined to comment through a spokesman for the Anne Arundel department. The spokesman said the assistant chief is dealing with “a personal family matter.”

How the courts handle the cases will vary. One federal judge on Thursday accepted motions filed by prosecutors and defense lawyers to dismiss charges against 10 defendants in a single drug conspiracy case, without having a hearing.

But in another case Thursday, U.S. District Judge Richard J. Leon rejected a move to dismiss drug charges against four defendants. Leon said he wanted more information about the government wanting to dismiss the case “without prejudice,” which would allow prosecutors to refile the charges in the future. Leon said “the government wants its cake and to eat it, too,” indicating he might accept a bid to dismiss the case “with prejudice,” barring further prosecution.

At the same time, Leon said the drug case was serious and he didn’t want to release the defendants without additional information, citing a need to “protect the public interest.” He said, “We’re going to move very carefully. Slowly. No sprinting.” The judge set an additional hearing for Dec. 10; defense attorneys said they would appeal to try to get their clients cleared sooner.

“I certainly understand the judge’s precautions and concerns,” Elita C. Amato, an attorney for defendant Darnell Jackson, said after the hearing. Amato, who said she has two other clients whose cases also could be dismissed in connection with the investigation, said she was surprised when she received dismissal motions concerning the FBI agent. “Obviously, there’s a lot going on here that we don’t know about,” she said.

Adam Goldman, Lynh Bui, Jennifer Jenkins, Clarence Williams and Matt Zapotosky contributed to this report.