A high school classmate of Tsarnaev confirms to BuzzFeed that @J_tsar is his Twitter account:
"I would confirm this as to being his Twitter account. Those users went to my high school," said Stefanie Gardner, of the Rindge class of 2011.
His profile picture:
Brobible also writes they've reached out to friends of Dzhokhar, who confirm he went by the nickname "Johar":
I DM'd with one Twitter user who is a student at the University of Massachusetts: Dartmouth (UMD) and who spoke a lot today, publicly on Twitter, about his relationship with him. He DM'd me back to say that he considers himself friends with the fellow UMD student. He confirmed that @j_tsar IS Tsarnaev's Twitter account, but he asked that he not be brought into this. At least one point this year the person I DM'd with is RT'ed by @j_tsar, so I don't think he's lying about the relationship.