wall of silence

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic Strict secretiveness maintained by the members of a group with respect to information which might be contrary to their interests, especially information concerning questionable actions by members of the group.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the secrecy of police officers who lie or look the other way to protect other police officers