Following that principle, Bin Laden wouldn't be responsible for 9/11, ISIS shouldn't have been restricted or attacked in any way, and the mafia should be fully legal. That would be absurd.
I only have the .gov version, but 20-30 rounds were fired, Song was convicted as the attacker apparently and Baumann and Morris were evidently armed as well (the woods gunmen). It wasn’t the whole group but doesn’t appear to be just one individual.
They were successfully tied together by the prosecution as conspirators to the satisfaction of juries by their shared mission, chats, organization, papers, safe houses, and prior meetings among other things.
Oh yes, how could we have forgotten the crucial detail that every single person in the group had their hands on the trigger.
Wait, that's because it didn't happen.
One individual fired their weapon. (And whether that's a murder is very questionable, but let's set that aside).
Nobody else should've been facing prosecution in this case.