All Alastor needed to do now was keep on the suspect's tail. That he could do. It boiled down to skill and the speed of the brooms, and in both cases, Alastor and John's were superior, given their training.
Because their subject had been startled by John's sudden appearance ahead of him, he couldn't pay attention to the Blasting Curse that Alastor shot off. The spell hit the bristles of the broomstick, which then caught ablaze, forcing the suspect to land.
There was a park by the river, and the suspect went down into the trees.