As reported by Gawker, the cached profile picture of the Twitter feed @J_tsar matches Boston bombing suspect Dzhokhar Tsarnaev, and the username matches his nickname, "Jahar."
CNN identifies the feed as Tsarnaev's.
Since the bombings on Monday, Tsarnaev or an impostor has been tweeting vaguely taunting ripostes, including a rebuttal to a story about a victim of the bombing: "fake story."
Before the attacks, the account's operator tweeted random thoughts about "Game of Thrones" and "Breaking Bad" as well as a message about media conditioning:
In January, he apparently encouraged those who were thinking about committing suicide to do so.