Federal prosecutors said Wednesday they will dismiss indictments against 28 defendants in District drug cases amid an investigation of an FBI agent accused of tampering with evidence, including narcotics and guns, according to newly unsealed court documents.

Fourteen of those defendants have already pleaded guilty and were serving sentences — one was a year into a 10-year term — and prosecutors said they can withdraw their guilty pleas and the charges would be dropped. A hearing is scheduled Friday in U.S. District Court for many of the defendants.

The stunning action by the U.S. attorney’s office came as authorities continue to scrutinize cases that the agent, assigned to a D.C. police task force in the FBI’s Washington Field Office, may have been involved with. The agent, who has not been charged criminally, has been suspended in what officials describe as a misconduct investigation.

One drug case that is unraveling involves an alleged gang that authorities said had imported heroin and cocaine from California into the District, Maryland and Virginia. Police said that they raided 26 homes and seized 11 pounds of drugs and that they listened in to thousands of hours of secretly recorded telephone conversations.

The alleged gang leader, 62-year-old Lester Pryor, and 13 others were indicted. Now, the government has indicated ­charges would be dropped against all of them.

“It was a very substantial drug case,” said Robert Jenkins, who represents defendant Anthony McDuffie, who had pleaded guilty to drug charges in the case and is awaiting sentencing. “I can only imagine the scope of the misconduct that occurred that led to something like this.”

The FBI and U.S. attorney’s office have confirmed that the investigation into the agent began the week of Sept. 29 and that authorities are still culling through cases that the agent may have been involved with. The Washington Post is not naming the agent because he has not been charged and neither he nor an attorney could be reached. He is being investigated by the inspector general’s office at the Justice Department.

Sometime in late September, the agent was found slumped over the wheel of his unmarked FBI vehicle near the Navy Yard, according to two law enforcement officials with knowledge of the case. Empty bags thought to have contained drugs were found in the car, the officials said.

The disruption to drug cases had been cloaked in secrecy, with many documents filed under seal that gave no hint anything was amiss. But U.S. District Judge Emmet G. Sullivan ordered the new information unsealed after The Post reported on the case Saturday. The judge said access to criminal proceedings “serves an important function of monitoring misconduct.” Prosecutors said they were already in the process of making the documents public.

In another of the affected cases, prosecutors filed court papers saying they would drop charges against 10 defendants accused of selling heroin and cocaine in the city. They also said that they would dismiss a 2013 case against four men who had been accused of selling heroin in D.C. neighborhoods.

Information about the investigation of the agent leaked after a judge last month ordered the release from jail and prison of more than a dozen felons and others with pending charges. Some of those cases are among those that prosecutors say will be dismissed, while others remain in limbo.

Officials say another case that could be affected involves a purported drug organization allegedly led by Juan Floyd. Thirty-three people were indicted in the case; none have been released.

Authorities have not described in detail how the agent’s alleged misconduct may have compromised the cases.

Defense attorneys said that they have not been given many specifics and described their clients as being in a holding pattern. But in the Floyd case, newly unsealed documents indicate that potential problems may stem from a search conducted Nov. 20 on a house and vehicle in Deale, Md., about 20 miles south of Annapolis.

One of the defendants in the alleged Floyd drug conspiracy was in court Wednesday, a routine bond hearing that revealed the complexities surrounding the investigation into the agent and its impact on pending court cases. It involved the 20-year-old daughter of the alleged kingpin, who is pending trial.

Brittany Floyd of Northeast Washington was arrested in a raid last year and charged with conspiracy to distribute drugs. Authorities said that they found heroin and scales used for weighing drugs in her bedroom; her attorney said that the drugs belonged to her father but that she was willing to plead guilty to a felony charge so that prosecutors would offer her father a plea deal.

Brittany Floyd had been released from custody as the drug case proceeded but had violated the terms of her release several times, including an arrest on charges of assaulting a police officer.

Her attorney, Jonathan Zucker, argued in court that prosectors had agreed to sentence her to probation on the drug case but that the investigation of the FBI agent was delaying an expedited hearing. He noted that prosecutors had agreed to drop the assault charge.

Zucker said that his client is a student and that it was unfair to keep her in jail because of what he called a “quagmire” involving the FBI agent.

Brittany Floyd’s mother, Tracey Luther, 49, said outside court that her daughter “is only being held because the FBI needs to clean up its own mess.”

Adam Goldman and Cheryl W. Thompson contributed to this report.