Other policeman is Angelo. For a couple of days leading up to the shooting, he had been threatened by the developers because he was trying to expose the fact that their site is toxic. Someone shot at him through a window and broke into his house, trying to shoot him. So when he was at the development site, he was really on edge.
When he heard the footsteps behind him and then the gun shot, he turned round and fired blindly behind him. It wasn't until it was too late that he realised he had shot Jack (who had been following Angelo, suspecting he was up to no good.)
Can you tell I'm way too obsessed with this programme?