Sounds fishy. Typically you do not get notified you have an active warrant till the police are acting on it. As well it came via mail, but was it certified mail/ did it have to be signed for? By them; do you mean the number they gave you or a number you know is legit? If it is a number they gave you, have you looked into it to see if it is a legit number with the local municipality? As they didn't list speed; did they list your license plate number, or just you driver license number?

A lawyer is a solid plan to verify, but some basic googling might help your piece of mind for the weekend https://rrlawaz.com/photoradar/