Alec Baldwin is involved in a freak accident when his prop gun, ended up causing an injury and a fatality.
Unfortunately these accidents are rare but happen once in a while, the most infamous one happened in 1993, when a prop gun ended up actually having live bullets, and ended up killing Brendan Lee, who was in his first and only movie, The Crow.
What's sad is this was the only child of Kung fu action film legend Bruce Lee, and The scene is still in the Crow, as it was how his character died before becoming the crowd's immortal role..
Oddly TV shows and movies are always filmed out of sequences of the script.
Editing is how they put the film together.
The beginning scene of most films, are always filmed last.
Our thoughts are with Alec and the families.
FYI, there will be no investigation as it was an accident, but like most accidents the families are guaranteed to sue, the people behind the movie to get some profit extra than just the work they were paid for.