Beats me. Maybe there weren’t specific threats, maybe they didn’t have enough evidence to arrest him. Maybe they were following him to catch others in a possible cell. But then they saw something (like a large knife) as he is about to step onto the bus and figured it might be a good idea to see what he is up to.
The actual shooting isn’t what has me questioning the entire episode. Big guy lunges at LEO with a knife, there will be consequences, usually deadly.
The entire episode does. This isn’t FBI SOP at all. Were these LEOs in uniform? And again, why no warrant for his arrest if he was so dangerous?