>> tomorrow all i want is peace while my son is being laid to rest. can you please, please, take a day of silence so we can lay our to rest? please. that's all i ask. and thank you. the deadly police shooting in missouri has brought into sharp focus the question of when it's the right decision to use a gun over something less lethal. cnn's renee marsh went behind the scenes with officers being trained to make that very call -- gun versus taser. >> police, don't move. >> don't move! >> drop the weapon! >> the officers were dispatched to a burglary if progress. >> taser, taser. >> this is only a drill. police in arlington county, virginia, arrive, guns drawn. within seconds, one officer swaps his gun for his taser. what triggered that decision?