Lot of internet lawyers here.

Simply put, in almost all US jurisdictions, a police officer cannot make a warrantless arrest for a misdemeanor that did not occur within his presence. The OP has described a Threat and an Assault and Battery. Both, are most likely misdemeanors, (at least they are here in Massachusetts). (They can in certain specific instances defined by statute, usually related to domestic issues, but that is not the case here.) It is also likely that the officer will not swear out a complaint, again, he has no first hand knowledge of the incident. But you can.

To the OP, you have his tag, you reported it. The investigating officer MUST file a report, and that report MUST be made available to you. Get it and go see an attorney. Only he or she will be able to properly advise you as to your rights and obligations in your state.

Good luck
zac

PS IAAL, but not yours, and the above is not legal advice. GO SEE AN ATTORNEY.