If you can see the agency names on the doors of the police cars, you can have your attorney call the agency in question to see what they want.
If you do feel compelled to communicate, go to a second-story window and open it. Talking through the window, ask the officer(s) knocking on the door to leave their contact information at the door and that your attorney will contact them shortly to discuss the matter in question. Close the window. By doing this you have not refused to cooperate, but rather have asked for legal counsel to represent you in all discussions with LEO's.
Details from the article are sketchy, but I'm betting the anti-military comments got the FBI involved and they showed up with some flunky social worker in tow. Because this guy engaged in a conversation with the officers/agents, he enabled the case worker to "observe" him and deem him "dangerous". Once he was deemed dangerous, I'm sure he started backpedaling and did not want them to arrest him, but by then it was too late. Having opened the door and engaged in discussion, he lost the protection of a warrant, since apparently.
Treat the government like a vampire. If you invite them into your house, you deserve to get what's coming to you...
Sorry for the grammatical errors above, I’ve gotten spoiled/sloppy by posting on WSJ.com, where there is an after-the-post edit feature....