The secrecy of police officers who lie or look the other way to protect other police officers WordNet 3.0 & Open English WordNet

The blue wall of silence, also blue code and blue shield, are terms used to denote the informal code of silence among police officers in the United States not to report on a colleague's errors, misconducts, or crimes, especially as related to police brutality in the United States. Wikipedia

A code among some police officers not to report misconduct by fellow officers, paralleling the "green wall" among prison correction officers Wikipedia Disambiguation

informal rule among police officers not to report on a colleague's errors, misconducts, or crimes, including police brutality Wikidata

Strict secretiveness maintained by the members of a police force with respect to information which might be contrary to their interests, especially information concerning questionable police actions. Wiktionary