You are supposed to follow them. You are just supposed to get close to them at the on foot portion at the end of the pursuit to "taze" and end it, but the detection is broken in this and other police job events.
I was following them, I was close to them, yet I fail.
And, well, I'm aware there are other police job events, but, I'm not really desperate for this one to be fixed, it's partially for the newbies not to have this type of experience, which is why I'm reporting it for it to be fixed.