Well, you first said the source was federal law enforcement official. It wasn't. Nor was it an official who was actually on the scene since the anonymous official was just "briefed" on the investigation, not personally party to it.
The problem is that a lot of people talk a lot of shit when this kind of chaotic event happens, including cops who heard something from another cop. It's a game of telephone. It may all turn out to be accurate, but for the moment I don't consider this a solidly sourced story.
ETA:
CNN says their source of the Joker story was indeed a federal law enforcement official. I'm still reserving judgement on this angle until the officers to whom he supposedly said this come forward.