Wilbert Stallings, 44, faces up to 10 years in prison after pleading guilty to a federal charge of conspiracy to violate civil rights. No date has been set for sentencing.
Mr. Stallings was expected to resign Monday from his position as a supervisor in the Atlanta Police Department narcotics unit, U.S. Attorney David Nahmias said.
Prosecutors said that in October 2005, Mr. Stallings and other officers used a warrant to raid an apartment, but when they found drugs out back they broke down the door of an adjoining apartment that they didn't have a warrant to search.
Nothing illegal was found in the adjoining apartment, either, prosecutors said.
Two police officers, Jason R. Smith and Gregg Junnier, previously pleaded guilty to state manslaughter and federal civil rights charges in the Nov. 21, 2006, shooting death of Kathryn Johnston. Mr. Smith and Mr. Junnier, who have left the police force, have been helping the government prosecute other officers for crimes unrelated to the shooting.
Prosecutors did not reveal who tipped them to Mr. Stallings' conduct. But the documents say Mr. Stallings was Mr. Junnier's commander.
A third officer, Arthur Tesler, was charged in state court with violation of oath by a public officer, making false statements and false imprisonment under color of legal process.
In January, a former Atlanta police officer pleaded guilty to his role in an extortion scheme that was uncovered during a probe of the Johnston case.
Under federal sentencing guidelines, Mr. Stallings will likely face much less than the maximum penalty. The exact amount was unclear. Prosecutors agreed to recommend that he be sentenced at the low end of the guidelines range if he cooperates fully. If Mr. Stallings' cooperation is substantial, prosecutors may ask U.S. District Judge Julie Carnes to sentence him below the guideline range.
The applicable sentencing range was not provided at Monday's plea hearing.
Ms. Johnston was killed in a hail of 39 police gunshots after plainclothes narcotics officers burst into her home using a no-knock warrant.