What? I thought this was over with. Okay... here's the 411:
Again, post 9/11, the Int'l Assn of Police Chiefs had a good idea. Based on the question, 'faced with a bomber... now what?' they thought to base police procedure on the physics associated with a suicide bomber type bomb. Smart so far. Well, professors at Yale & the US Naval Postgrad school undertook experiments and to quote them, "the conclusions were quite unexpected". Here,you read:

"It turns out that very few people are killed by the concussive force of a suicide explosion; the deadly weapon is in fact the shrapnel . The explosions, though, are usually not powerful enough to send these projectiles all the way through a human body, which means that if your view of a suicide bomber is entirely obscured by other people at the moment of detonation, you are much more likely to escape serious injury. Because of the geometry of crowds a bomb set off in a heavily populated room will actually yield fewer casualties than one set off in a more sparsely populated area; the unlucky few nearest to the bomb will absorb all of its force."
further (sorry, more reading):
"calculations demonstrate that causeing people near the bomber to scatter or hit the deck would make things worse - as a packed crowd ran away from a bomber or dropped to the ground, the circle of potential victims around him would get wider and thus more populous, and more lives could be lost."
So, according to both the Pentagon findings (shooting a bomber is more likely to cause a detonation than not); and the Police Chief Assn findings (don't tell people to hit the deck) the air marshall's actions were 100% wrong.
But don't feel bad, because DESPITE all the studies proving to the opposite, you know what the 'powers that be' decided? Get this:
"The International Association of Chiefs of Police issued guidelines this year suggesting that police officers who find a bomber in a crowd should fire shots into the air".
I feel safer! how about you?