I bet they used unload guns for FOF and pointed them at each other and after lunch or such someone 'forgot' they had loaded their gun at lunch time.
Near Dallas about 8 years ago they had a swat exercise at a highschool in the summer. One of the trainers during lunch loaded his gun with real ammo (they had been using simulation rounds during practice.) Well he demo'ed with a shotgun at another trainer and when he missed his shot he went to his handgun and shot the other trainer dead.
And that is why I do not like using real guns for ANY FOF training, no matter how many times they are checked to make sure they are empty.
People forget, including me, so at home I have dummy guns for all my training.
"The government has confiscated all of our rights and is selling them back to us in the form of permits."