He worked for the police/government as a paid 'informer'. [='informant']
(..) What do we do with this cop? They want to have a meeting with me, right? It will be me, McCluskey and Sollozzo. Let's set the meeting. Get our informers to find out where it's going to be held. We insist it's a public place. A bar, a restaurant. Some place where there are people, so I feel safe. (..)1972 The Godfather