He didn’t get in. The building whose roof he fired from was outside the security perimeter. He didn’t need to go through the security checkpoint to reach it.
So when did they label him as suspicious? There was a report that said that the distance finder was caught in the checkout but they couldn’t stop him from coming in because it wasn’t considered contraband but he was labeled as suspicious.
But then, the story has been changing all along so who knows what really happened.