In his briefing on Wednesday, Mr. Thompson was careful to disclaim any political agenda, saying the process was focused on one officer, not the entire Police Department or any national debate.

“This case has nothing to do with Ferguson or Eric Garner or any other case,” he said.

But it inevitably became part of the national push for criminal justice reform and a defining episode for Mr. Thompson, who is black. Elected as a fresh voice of change, he is also the son of a police officer, and in his remarks on Wednesday, he sought to affirm both his mandate to remake the office and his obligation to work with law enforcement. He spoke of his obligation to Mr. Gurley, his obligation to Officer Liang and his obligation to the people of Brooklyn.

And he offered strong praise for the police in general and the bravery of the other officers who responded to reports of shots fired on Nov. 20 — believing they were facing a gunman before realizing the gunfire came from one of their own.

Minutes before, Officer Liang had been on a routine patrol. He approached the stairwell door with his gun in his left hand and a flashlight in the right. Pushing the door open with his right shoulder, he turned to his left and fired, Mr. Thompson said.

“In order to fire the gun, you need a certain amount of pressure to put on the gun,” Mr. Thompson said, suggesting he did not believe the shooting could be accidental.