At this point, we know very little, but with what we do know at the moment - for this, you’d need:
someone willing to be taken out as the shooter, willing to or being tricked into shooting blanks, or an extremely elaborate plan to switch in a body (you also have to acquire and prepare somewhere) in front of witnesses and potential cameras
someone without blanks firing to explain the other bystanders dead from bullets
complete control over the subsequent investigation, to make sure no one looks into it properly, which would make all the people involved in the know
It is still an extremely, extremely unlikely scenario