Total speculation, but what if they found evidence he'd visited that alleyway before, to scope it out (or even, went back after). Or, careful questioning of the witnesses to his interactions that night might reveal he was steering Tess in that direction, or in a way to ensure they were alone very late. Or new witnesses that he'd tried to attack women in a similar way in the past. Or careful, frame by frame analysis of the suveillance cameras to show he used some force to pull her into the alleyway right her after she called for an Uber, and that she tried to resist. Various ways to build a case that he was behaving in a predatory way that night.