Jurors found Dzhokhar Tsarnaev, guilty on all 30 counts stemming the April 15, 2013 bombing of the Boston Marathon.
This bloody note, written by Tsarnaev himself, likely helped them arrive at such a stark verdict.
Earlier in the trial, Tsarnaev's lawyers and a terrorism expert serving as a prosecution witness argued over whether the defendant was paraphrasing Al Qaeda propaganda in a note he left four days after the deadly attack.
While hiding in a boat hours before his arrest, Tsarnaev scrawled a note reading, in part, "we Muslims are one body you hurt one you hurt us all," a message that counter-terrorism expert Matthew Levitt said was similar to extremist writings found on his computer.
Defense attorney David Bruck asked Levitt if it was not possible that Tsarnaev, now 21, had heard those words from his older brother, Tamerlan.
"Could other people have hit these points too? Maybe in that same verbatim language? Maybe," Levitt acknowledged at U.S. District Court in Boston. "Could that have contributed to the radicalization? It could have."
Tsarnaev is accused of killing three people and injuring 264 with a pair of homemade pressure-cooker bombs at the race's crowded finish line on April 15, 2013, and with fatally shooting a police officer three days later as he and 26-year-old brother, Tamerlan, tried to flee.
Tamerlan died hours after the shooting, following a gunfight with police.
Defense attorneys opened Dzhokhar Tsarnaev's trial March 5 admitting he committed the crimes, but are seeking to spare him the death penalty by painting him as a mostly normal American kid who fell under the spell of his older brother.
Levitt, a senior fellow at a Washington think-tank and a former U.S. intelligence agent, told jurors he did not know if the materials found on Tsarnaev's computer, including sermons by U.S.-born Al Qaeda figure Anwar al-Awlaki and issues of Al Qaeda's "Inspire" magazine, were put there by Tsarnaev or by someone else, such as his brother.
But Levitt contended that it mattered not where Tsarnaev got the materials but only that he had them and that there were "direct parallels between the materials on the devices and the defendant's own writings."
"At a certain point it almost doesn't make a difference where you got them, if you got them and internalized them," he added.
Tsarnaev's words in the note, "Now I don't like killing innocent people but due to said (bullet hole) it is allowed," resembled lines from an editorial in Inspire, Levitt said.
Levitt, who is paid $450 an hour by the prosecution, also acknowledged under cross-examination by defense attorneys he speaks neither Arabic nor Russian and is not an expert on Islam.
The Tsarnaevs, ethnic Chechens, immigrated to the United States from Russia a decade before the attack and settled in Cambridge, Massachusetts, just outside Boston.
Separately, Khairullozhon Matanov, a citizen of Kyrgyzstan, is due to plead guilty on Tuesday on charges of lying to investigators probing the bombing. The man was a friend of the Tsarnaevs but played down how well he knew them.
Reuters reporting by Elizabeth Barber.
(Additional reporting by Richard Valdmanis; Editing by Scott Malone and James Dalgleish)