Apparently Tamerlan was the ring leader and became increasingly bitter towards the American government in the months leading up to the attack. Also, it is believed that Tamerlan was run over by Dzhokhar during the shootout with Boston police, killing him.
But the most interesting part of all of this new information is that Dzhokhar was on the Dartmouth campus everyday after the bombing until late Thursday. He attended classes and even dorm parties. Crazy.