According to the indictment, Mr. Gladstone was at dinner with another officer on March 26, 2014, when he received a cellphone call from another police sergeant referred to as “W.J.” The sergeant had just deliberately run over a suspect referred to in the indictment as “D.S.,” the court documents said.

In response, Mr. Gladstone retrieved a BB gun from the trunk of his police car and drove with the officer he had been having dinner with to the scene, prosecutors said. Once there, Mr. Gladstone dropped the gun next to a pickup truck near where the suspect was lying injured on the ground, the indictment said. Mr. Gladstone then told the sergeant, “W.J.,” that the gun was near the truck and instructed the sergeant to have another officer search it, the indictment said. Mr. Gladstone then left the scene with the officer he came with, it added.

The sergeant referred to as W.J. then told another officer to move the BB gun under the truck, closer to the victim, prosecutors said. It was eventually recovered by the Baltimore Police Department’s crime lab unit and the suspect was subsequently charged with possession, use, and discharge of a gas or pellet gun — as well as a number of drug offenses — based on a false statement of probable cause written by W.J. in another officer’s name, the indictment said.

The suspect was detained on those charges for about a week; the charges were dismissed about eight months later.

According to the indictment unsealed this week, Mr. Gladstone — who retired about two months after the task force members were charged — told the officer with whom he had been having dinner that if the officer was questioned by federal law enforcement officials about the 2014 episode, the officer should lie and tell them that they had been deployed there to provide “scene security.”

This year, Baltimore’s mayor tapped Commissioner Harrison to become the city’s fifth police chief in four years, and asked him to try to solve the many problems that chased most of the others from the job. Among the tasks was building trust among residents who widely view the department as racist, corrupt and indifferent.

In a statement about the indictment of Mr. Gladstone, he said the allegations “speak to a culture that I am here to change.”