12:18 PM PT -- TMZ Sports now has video showing Conor picking up a dolly and smashing the side window of the bus.
We spoke with Dana White ... who tells us Michael Chiesa was "cut to pieces" in the attack and is currently hospitalized for his injuries. It's unclear if he will fight as planned at UFC 223.
White also says Rose Namajunas was also on the bus and was very shaken up during the incident -- she almost broke her hand.
We asked if Dana will fire Conor as a result of the incident and he told us, "This is bigger than us firing him.