I agree with your assumption. How do you get weapons past perimeter security on Inauguration Day? By having them already stashed inside the perimeter.
On 9/11, those box cutters were not smuggled in by the hijackers. Box cutters were found on grounded planes which had not been boarded — indicating that they were stashed there ahead of time by people on the airline maintenance staff.
Most people do not know this and are very surprised when I tell them.