Boston Marathon Bombing Trial

In this courtroom sketch, defense attorney Judy Clarke is depicted addressing the jury as defendant Dzhokhar Tsarnaev, right, sits during closing arguments in Tsarnaev's federal death penalty trial, April 6, 2015, in Boston.

FILE - In this courtroom sketch, Dzhokhar Tsarnaev, center, is depicted between defense attorneys Miriam Conrad, left, and Judy Clarke, right, during his federal death penalty trial.

Judy Clarke, left, and David Bruck, defense attorneys for convicted Boston Marathon bomber Dzhokhar Tsarnaev, will try to prevent his getting a death sentence. They're shown at the federal courthouse in Boston, Massachusetts, April 6, 2015.

A police dog sniffs near news cameras outside the U.S. Courthouse during the first day of the Dzhokhar Tsarnaev trial, in Boston, Massachusetts, March 4, 2015.

A still image captured from surveillance video at the Boston Marathon shows the scene moments before a second bomb exploded as a man, marked with a circle by prosecutors (top R) and identified by them as defendant Dzhokhar Tsarnaev, moves rapidly away fro

A still image entered as evidence shows the boat where Boston Marathon bombing suspect Dzhokhar Tsarnaev hid after a gunfight with police in this handout photo provided by the U.S. Attorney's Office in Boston, Massachusetts, March 17, 2015.

A blood-stained message that prosecutors say Boston Marathon bombing suspect Dzhokhar Tsarnaev wrote on the inside of a boat is seen with bullet holes in an undated evidence picture shown to jurors in Boston, March 10, 2015.

FILE - A portion of a pressure-cooker bomb recovered from a blast site at the Boston Marathon bombing is displayed for the media in a conference room.