I believe that you are referring to a case called Andrew v. Clark. This case involved Michael Andrew, a Baltimore police officer, who was demoted when he gave a Baltimore newspaper a memo that he had written in which he was critical of police actions in the killing of a Baltimore man. Andrew argued that his demotion was a violation of his First Amendment right to freedom of speech. In this case, Andrew ended up winning. The 4th Circuit Court of Appeals ruled in Andrew’s favor, at which point the City of Baltimore chose not to appeal.
Andrew was critical of the way that police had handled an incident in which a 78 year-old man was killed. The man had killed the manager of his apartment and had barricaded himself in a room. The police stormed the room instead of trying to negotiate and the man was killed. Andrew felt that the storming of the room was premature. Getting no response to his memo from within the department, he released it to the press.
The 4th Circuit held that the demotion violated Andrew’s rights. They also held that it would make it harder for the media to scrutinize government actions if such whistleblowers could be punished. For these reasons, Andrew prevailed in his suit.