inspector general

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An officer with general investigative powers within a civil, military, or other organization.

from The Century Dictionary.

• noun An officer charged with the oversight of some system of inspection, as that of an army, a class of public works or of machinery, etc.

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

• noun a senior official responsible for inspecting conformance to the standards in a civil or military organization

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

• noun a military officer responsible for investigations