The people who work in any district attorney's office, anywhere, are in constant contact with law enforcement officers—answering questions, making arrangements, taking messages. Anybody gutsy enough to call and claim to be a police officer, sheriff's deputy, or whatever will likely be taken at his word. Unless it's obvious that he doesn't know the terminology, or if he's nervous and stumbles over his words, or in some other way doesn't sound authentic, he may not even be asked a single question to verify his claim. That's exactly what happened here, with two different workers.