Yes, Tamerlan was dead before this guy was arrested.
This arrest was supposed to be the OTHER brother.
Yep. Tamerlan was his brother Dzhokhar's Speed Bump, and this innocent citizen was the Watertown PD's speed bump, helping Dzhokhar's temporary escape. It's called the fog of war.
Now, only Joker survives, but the President had him Mirandized, because he pledged to "stand with the Muslims should the political winds shift in an ugly direction".