informer
noun /ɪnˈfɔːmə(r)/
  /ɪnˈfɔːrmər/
- a person who gives information to the police or other authority- The bomber turned (= became) police informer to escape prison.
- One of the gang members had turned informer.
 Oxford Collocations Dictionaryadjective- police
 - become
- turn
 - a network of informers