- there was no security checking at the concert
- there was at the stadium.
Commentators observed that the terrorists "had done their homework", e.g., had gone through dry runs to check out the security practices.
Given this, the perpetrators had checked out the stadium and found it to be a "soft" target.
Someone challenged a bomber. That person or persons saved more than one (the President's) life.
That person was almost certainly a casualty.