How can I find out who a licence plate belongs to?

I was just driving home from my college and a man starting yelling ulal profanities at me at the red light. As I drove away he followed me. I called 911 and was on the phone with them for about 3 minutes. The car than speed of. I did get his licence plate and when the police arrived they said that the plate number is not registered in this state. Unfortunately I do not know what state the plate is from. I live close to the border of Maryland and PA. I guess there is nothing I can do at this point. I was dressed in a black sweater, I do not know why he singled me out.