An investigation into possible misconduct by an FBI agent has forced authorities to quietly release at least a dozen convicts serving prison sentences for distributing drugs in the District and its suburbs, according to law enforcement officials, court documents and defense attorneys.

In addition, several suspects awaiting trial on drug charges and a man convicted but not yet sentenced have also been freed. Officials said more cases­ that could involve the agent are under scrutiny, including one involving 21 defendants.

None of the suspects or felons have had their charges dropped or convictions overturned. Most are on home detention in what many of their attorneys describe as a holding pattern, awaiting the outcome of the investigation into the agent, who was assigned to a D.C. police task force.

The scope and type of alleged misconduct by the agent have not been revealed, but defense lawyers involved in the cases­ described the mass freeing of felons as virtually unprecedented — and an indication that convictions could be in jeopardy. Prosecutors are periodically faced with having to drop cases over police misconduct, but it is unusual to free those who have been found guilty.

A law enforcement official speaking on the condition of anonymity said the agent has been suspended indefinitely. The agent has not been criminally charged.

The U.S. attorney’s office for the District said in a statement Friday that it is “conducting a case-by-case review of matters in which the FBI agent at issue played some role.”

“We have already begun taking steps to address this issue and are committed to doing everything that is necessary to preserve the integrity of the criminal justice process,” the statement said.

The decision to release the defendants and convicts was made with little or vague public notice. In one case, eight convicts and one defendant who pleaded not guilty were released to home detention Monday, with no indication publicly filed in court. One man who had served nine months of a 10-year sentence was sent back to the District from a federal prison in North Carolina. In another case, a cryptic court document ordered the “immediate release from incarceration” on Oct. 17 of four convicts and others with pending trials for the “duration of a current investigation.”

“I’ve never, ever seen something like this before,” said Robert Lee Jenkins Jr., a lawyer from Alexandria who is representing Anthony McDuffie, 50, who pleaded guilty to a drug conspiracy charge and has been released pending sentencing. “It suggests to me that whatever is going on is very significant.”

Said another defense lawyer, Gregory English, whose client was released as he awaits trial: “This is stunning.”

Among the cases was one that the head of the FBI’s Washington Field Office highlighted in a news release last year as the culmination of a year-long investigation that police said traced heroin and cocaine from suppliers in California to street dealers in the District, Maryland and Virginia. In all, 11 pounds of the drugs were seized in the searches of 26 homes and storage facilities, along with five guns. Affidavits filed in the case show that police listened in on cellphone calls during money and drug drops at a Metro station in Northeast and a barbecue restaurant in Northwest and that the dealers frequently exchanged bundles of cash totaling as much as $85,000.

Police also alleged that the group was involved in identity theft involving hundreds of credit cards, Social Security cards and driver’s licenses. U.S. Attorney Ronald C. Machen Jr. hailed the indictments last year and said police were “able to remove guns, drugs and dangerous people from the streets and take another step toward making our community safer.”

Now, all 13 people indicted — including five who pleaded guilty — are free from jail or prison. One is the alleged ringleader, Lester Pryor Jr., 63, who is awaiting trial.

English, who is representing Brandon Beale, 58, in the Pryor case, said he was planning to fight the charges­ before the revelations. He said Beale, who has been freed pending trial, was an addict, not a distributor, and he plans to argue that authorities targeted “what they thought was a group of major dealers who turned out to be a very small one,” and that the others indicted were mostly users.

“It cost the FBI a lot of money to run a wiretap, and they didn’t get what they wanted,” said English, a former federal prosecutor. He said his case is “in a holding pattern” but added: “I’d be surprised if the prosecutor proceeds with the case. If they do, our case has become infinitely stronger.”

In a statement, the spokesman for the FBI’s Washington Field Office said allegations regarding the agent first surfaced the week of Sept. 29 and involved “possible misconduct.” The statement said authorities “took immediate steps to address the incident” that included notifying prosecutors who had cases­ involving the agent.

The Justice Department’s inspector general’s office is leading the investigation into the agent. FBI officials declined further comment, and D.C. police declined to comment.

Some earlier cases­ of police misconduct in the District have had sweeping implications. In 1987, authorities dropped 300 pending criminal cases­ amid an investigation into D.C. police officers skimming drugs and money seized during raids. In that same case, convictions were dismissed against 12 who had already been sentenced.

Authorities said they were looking into virtually every case in which the agent, who has not been publicly identified, was involved. It was unclear what role the agent has had in the cases thus far.

One of those cases involves alleged drug kingpin Angel Costello and 11 others indicted with him on drug charges, according to officials familiar with the investigation. Although there was no notice publicly filed in U.S. District Court indicating a change of status for the defendants, the Federal Bureau of Prisons inmate locator shows eight people convicted in the case were freed Monday — months and years before completing their sentences. Costello has pleaded not guilty and is awaiting trial.

In the Pryor case, a three-page order in the public court file calls for the release of five defendants who pleaded guilty, four of whom had already been sentenced. U.S. District Judge Reggie B. Walton describes the “court’s authorization to order the [defendants’] immediate release from incarceration for the duration of a current investigation being conducted by the government that resulted from its acquisition of new evidence.” He added that the release was in the “interest of justice.”

Several defense lawyers interviewed said they are in a difficult position because they know little of the allegations involving the agent. Many of the defendants have what are called “status” hearings in the next few weeks during which lawyers said they hope to learn additional details.

Prosecutors could still move forward with some or all of the cases but would face an additional hurdle of proving that any misconduct on the part of the agent did not have any impact on the charges. Officials said decisions will be made on the merits of the case against each suspect.

Defense lawyer Ron Earnest, who is representing James “Sweet Baby James” Burkley, 59, said he readily recommended that his client accept a seven-year prison sentence for his alleged role in the Pryor drug case. Burkley pleaded guilty Sept. 23, a week before the FBI said the alleged misconduct became known.

Earnest, who has an office in Riverdale, Md., said that in late October his client called him from the D.C. jail, where he was awaiting placement in a federal prison in North Carolina. He said Burkley told him that co-defendants in the case were being released, including the alleged kingpin.

Earnest said he called prosecutors and was soon summoned to court. He said the judge told him and other defense lawyers that “something was wrong with the investigation” and “everybody would be released, including the people who pleaded guilty.”

Earnest said his next step is to file a motion asking that Burkley’s conviction and sentence be vacated so “that my client can get back to his life. . . . But it’s also possible that the government doesn’t want to invalidate the conviction.”

Clarence Williams, Jennifer Jenkins, Justin Jouvenal, Keith L. Alexander, Ann E. Marimow and Sari Horwitz contributed to this report.