First of all, you of course need to leave the mill so, go Southwards to return to the Loading Bay Depot [Loading Bay]. Note that there are new enemies in the mill's first location. Just like before, it would be a good idea to organize a nifty surprise attack, e.g. by sending a propane tank towards the biggest enemy group. Remain for one more moment in the loading bay and spread the explosive gel over the big tank located on the right. Detonate the gel, which will unlock a side mission connected with the Black Mask (the description of this mission can be found in a separate chapter of this guide)
You can now return to the Industrial District. After you listen to the conversation with Alfred go towards the Gotham Pioneers Bridge. This mission is exceptionally easy because it requires you to use the hook and the gliding option to reach one of the bridge's upper balconies located on its central pylon. These actions will result in the beginning of the process of tracking the Electrocutioner.