"...unknown bureau agents..."
It's seems there's a lot of that lately. What are these people? The secret police? I thought we were rid of them when Hitler was defeated. I want to see a business card from these guys. I want to see not only their office address but their home address and their social security number. They work for me and I'm going to insist on accountability. Then, if one says something that is in violation of his oath to protect the Constitution, he can be deported as fraudulently naturalized.
I would imagine the agents were wearing coveralls just like everyone else. Probably even had FBI written on the left front and in big lettering across the back. I am confident that the NYPD did check them for badges and ID cards.
Once they put on the coveralls, I doubt that they felt any more identification was necessary.