WATERTOWN, MA - APRIL 19: Police walk down School and Walnut Street on April 19, 2013 in Watertown, Massachusetts.
This image provided by the Boston Regional Intelligence Center shows Dzhokhar A. Tsarnaev, one of the suspects in the Boston Marathon bombings.
This image provided by the Boston Regional Intelligence Center shows Dzhokhar A. Tsarnaev, identified by the FBI as suspect number 2, in the Boston Marathon bombings. Authorities say Tsarnaev is still at large after he and another suspect - both identified to The Associated Press as coming from the Russian region near Chechnya - killed an MIT police officer, injured a transit officer in a firefight and threw explosive devices at police during their getaway attempt in a long night of violence into the early hours of Friday.(Boston Regional Intelligence Center)