How do I get my impounded bike back from the police?

So me and my brother recently bought a dirt bike 5/24 we have the bill of sale and registration from the first owner but the only thing we didn't do was register it as of yet but we wanted to see how it would run so we took it for a quick ride no reckless driving or anything out of the ordinary but we also live in the city on public streets (we know we wasn't supposed to do it but we still did it please save me the lecture) and a cop came and stopped us and told us he could lock us up for Doing something like this mind you this is our first time ever attempting something like this so I was thinking we would get a warning but I guess not so the guy also goes on with asking us where we work and whatnots our names and address he didn't state his name to us or tell us what we can do to get the bike back now I know the guy thinks the bike is stolen cause he ran the numbers and nothing came back on it cause we PURCHASED IT LEGITIMATELY but he still thought it was stolen. He also didn't fine us or give us anything we could use to follow up on this situation just a number and told us to call them I was just wondering what i needed to know or do to get the bike back