If you didn't find all policemen alive you need to get the silver card. Here's a guide from someone else on how to get it
"I found it you have to fly to Fiore Mansion and surf over to the left. Then you have to walk up the stairs and go into Citrine Mountain and surf and waterfall down. Next, you use strength and go back up and you can slide over the ice. After that, you can get skuntank and the silver card. Finally, you go back to his gym and use the card and you get field effect readout 11 and a toxic orb."