As has been stated, Geller only had a volunteer attendance. Nobody was in danger unless they wanted to be in that situation. The security detail was voluntary as well, and had intel from the FBI this might occur two days earlier.
Those are the facts.
More facts, this may have been previously know about and prevented.
As previously stated n debated, there could have been a McVeigh type of bomb that would have not only blew away and killed the volunteer contestants but it could easily destroy buildings n property and businesses that didn't volunteer.
It could have been much worse.