Late yesterday evening as the entire event was winding to a close, I sat on @BostonGlobe's twitter page while listening to the audio stream. Every tweet they put out was nearly identical to what was coming from the scanner audio, prefixed with "A source says ...". The reason it was only "a source" was because no one except the dispatcher and other cops on scene knew exactly who was saying what. It's just some voice coming over the radio who is assumed to be a cop.