It's not like they couldn't tap his phone and his computer either if he is suspected of hatching some plot against the witnesses. That sounds pretty far fetched anyway. He's not some big mob boss with a crew of people sworn in blood to be loyal to him. Who would collude with him? Some of his old retired cop friends? They're going to risk prison to help him beat the rap of shooting someone in a movie theater? That's a crazy idea.
Have you never lived in a city? I have; and many of the cops are as crooked as the crooks.