Quote Originally Posted by c6h6o3
If you were not on government property, he can't tell you to do anything. Even if you were on USPS (which is only quasi-governmental, BTW) property I would question his jurisdiction unless he's part of the Federal Protective Service, which provides security for Federal Buildings.

I work at a US Government facility. If we observe a breach of security we call the FPS. I'm a contractor, but not even the Fed employees can take any action on their own. Unless I'm challenged by a uniformed officer with a badge, I'm very uncooperative.

I would've continued photographing and told the man to kindly bugger off.
It could have been a postal inspector. They are plain clothed and are federal law enforcement agents with the authorty to arrest. I would think he would have shown a badge or federal ID.

Postal employees are instructed to inform either the postal police or the inspectors if there is suspicious activity on or in federal property. They should not act on their own.

It is too bad, it looks like a very nice building to photograph.