The drug ring served the District, Maryland and Virginia. It had a network of dealers, suppliers and stockpiles of heroin, cocaine, marijuana and methamphetamines.

It also had another very important asset: a Prince George’s County police officer who knew that authorities were closing in, according to a federal grand jury indictment unsealed this week.

The indictment alleges that Cpl. Vanessa Edwards-Hamm, a longtime narcotics officer and a member of a federal drug enforcement task force, tipped off a member of the ring when FBI agents were listening to the person’s phone calls and monitoring their text messages and e-mails.

Edwards-Hamm, 38, was charged in federal court in the District with one count each of tampering with documents or proceedings and unlawful notice of electronic surveillance. If convicted, she could face a maximum of 25 years in prison.

“In this case, a police officer sworn to uphold the law is accused of undermining law enforcement by revealing covert electronic surveillance,” said U.S. Attorney for the District Ronald C. Machen Jr., who along with the FBI and D.C. police named 16 defendants in the drug conspiracy on Wednesday.

Court filings did not specify which of the defendants the officer allegedly tipped off. But one of those who had his calls recorded was Edwards-Hamm’s brother, Mark Edwards, 39, of Capitol Heights.

In a search of Edwards’s home in April, FBI agents found plastic bags containing heroin, a digital scale and ammunition, according to a detention memo filed by the’s office. When agents played for Edwards recordings of conversations between him and an alleged heroin dealer, prosecutors said that he acknowledged working as the “middleman” but “did not consider himself a ‘big guy.’ ”

Edwards, who prosecutors said has a pending drug possession charge in a separate case, waived his right to a detention hearing in U.S. District Court on Wednesday. His attorney Dwight Crawley declined to comment.

Edwards-Hamm pleaded not guilty Monday and was released on her own recognizance.

The arrests came after a year-long investigation by the FBI and D.C. police that involved wiretaps of cell phones and surveillance, and revealed what prosecutors said in court filings was a “large-scale network which redistributed illegal drugs on a regular basis.”

According to those filings, sometime between February and April, Edwards-Hamm allegedly knew that the FBI was monitoring the communications of a suspect and alerted that person “in order to obstruct, impede, and prevent the grand jury’s investigation,” according to the indictment against Edwards-Hamm.

Edwards-Hamm, who could not be reached for comment Wednesday, was hired by Prince George’s County police in 2004. For the past several years, she worked in its narcotics enforcement division. She was given a prestigious post on a Drug Enforcement Administration task force, according to a law enforcement official and a relative familiar with her career. The official, who spoke on condition of anonymity to freely discuss the case, said Edwards-Hamm was not investigating the alleged drug ring, but was privy to information about the wiretaps as a member of the federal task force.

Edwards-Hamm was placed on administrative duty when police learned of the federal investigation, officials said, and was suspended this month without pay.

Prince George’s County police spokeswoman Julie Parker stressed that the county department worked with the FBI to arrest Edwards-Hamm. “She was in cuffs,” Parker said.