A Maryland man has pleaded guilty in federal court to distributing large amounts of cocaine in the District, a conviction that comes in one of the cases affected by an FBI agent accused of tampering with drug and gun evidence.

A plea agreement filed in U.S. District Court says that Timon Otis Sandidge, 38, who lives in Temple Hills, forfeited his right to appeal based on the outcome of the still pending investigation of the agent, Matthew Lowry, who has been suspended from the FBI’s Washington field office.

The court papers say that Sandidge also cannot challenge his conviction based on any “newly discovered evidence of alleged misconduct or into alleged deficiencies in the FBI’s evidence control procedures.”

Lowry was found slumped over the wheel of his unmarked FBI car on Sept. 30, with what court documents describe as opened packages of drugs and other evidence checked out from his office or taken from crime scenes. Authorities have been working for more than two months to determine whether his alleged conduct would impact the outcome of criminal cases in which he was involved.

Federal prosecutors have already dismissed cases against 28 defendants in four drug cases in the District, saying that the alleged misconduct was too great a burden to overcome. Authorities have said that 150 other defendants have cases affected by Lowry, but they argue his role in those was minimal or that none of the alleged tainted evidence is being used in their cases.

Still, dozens of criminal cases before several judges remain in limbo, as defense lawyers contemplate challenging charges filed against their clients based on what attorneys now refer to in court as the “Lowry issue.” It is complicated by the fact that no charges have been filed against Lowry and investigators are trying to maintain the secrecy of their investigation into the agent. Prosecutors have said they hoped to conclude that investigation by the end of the year.

Sandidge is the first defendant in a case affected by the Lowry investigation to plead guilty; authorities say the conviction shows that many cases involving the agent can be salvaged, and that not every case will be dropped.

U.S. District Judge Emmet G. Sullivan sentenced Sandidge to six years in prison, followed by five years of supervised release and 100 hours of community service.

In a statement, the U.S. attorney’s office said Sandidge was one of more than two dozen people charged in 2012 in connection with a joint FBI-D.C. police investigation into a cocaine network that sold drugs from September 2010 through March 2012.

Sandidge admitted to buying more than four pounds of cocaine from a supplier selling the drug to dealers. Police say that during a search of his house in Maryland they found a semiautomatic pistol in a dresser drawer in his bedroom and $3,000 in a safe. Sandidge has previous convictions in Maryland and in the District for drug distribution and handgun possession.

Sandidge’s attorney, A. Eduardo Balarezo, represents one other client affected by the Lowry case and has fought in court for judges to unseal documents describing the alleged misconduct so that he can better advise his clients on how to proceed. He has also raised questions about the FBI’s evidence-control procedures and whether criminal cases beyond those worked by Lowry could be affected.

Balarezo said that Sandidge, with a different attorney, had pleaded guilty before the Lowry case broke and had agreed to serve 11 years in prison. Balarezo said he was able to get the plea withdrawn and renegotiate, using Lowry as leverage, for a lower sentence.

He said Lowry helped execute a search warrant on a co-defendant’s house but did not appear to have any connection to Sandidge. All other defendants in this drug case have pleaded guilty.