I chased down a mugger once. I heard a yell from across the street. Looked out and saw two kids pulling a bag from a woman.
I saw them run in two different directions. I jumped into a pair of shoes and rushed outside to follow one at a distance.
The kid was nervous, kept looking around - which made it easy to stay hidden from him even though I was getting closer.
He looped up and down a couple of blocks, and I followed until I saw police near the Capitol. Then I closed the gap.
On the same block as the police I grabbed him and yelled for help. He wiggled free and ran straight toward the police car.
The police detained both of us. Kid's story was that I had attacked him. Mine was - as above. Radio check proved my story.
The other robber had been caught too, so he was brought to us. Then the victim was driven past to ID them - confirmed!
A couple of weeks later I was at court to testify. Spent most of the day in an empty room alone. (I had the time.)
At the end of the day I was dismissed. I asked what had happened, why I hadn't been asked to testify.
It turns out the victim had never pressed the charges - but between being caught and my observations he might still be.
He and his lawyer had COUNTED on me not taking the time to tell my story, assuming the charge would evaporate after that.
The kid was in much bigger trouble for other things too. Without this charge he might have been merely "troubled".
With "my" case he had a lot more to deal with, and he (and his lawyer, and his mother) couldn't ask the court to ignore it all.
When I showed up, even without a cooperating victim, they had to re-evaluate, and decided to bargain.
In the end, he wasn't actually charged with this mugging - the other kid had the bag and this kid was an accomplice.
But facing this charge on top of whatever else he had done, he was, to some degree, stopped.