WASHINGTON — 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 of Columbia 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.

The scope and type of alleged misconduct by the agent has 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 in the wake of 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 of Columbia 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.”

“I’ve never, ever seen something like this before,” said Robert Lee Jenkins Jr., a lawyer from Alexandria, Virginia 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.”

Added 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 streets dealers in the District of Columbia, Maryland and Virginia.

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 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.

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