A law enforcement source also told CNN on Thursday that the message said Dzhokhar, 19, would not miss his older brother Tamerlan Tsarnaev, 26, – who had already been killed by police on April 18 – because he would soon be joining him as a martyr in paradise.
"When you attack one Muslim, you attack all Muslims," read the note, scrawled with a marker on the interior wall of the boat's cabin, according to CBS News senior correspondent John Miller.
The note, which is "certainly admissible" as evidence at trial, referred to the Boston victims as "collateral damage" in retribution for U.S. military action in Afghanistan and Iraq, Miller reports.
Tsarnaev is currently being held in a federal prison hospital in Massachusetts, charged with using a weapon of mass destruction when two bombs exploded near the Marathon's finish line, killing three people and injuring 264.
He could face the death penalty.
Lucas Jackson / Reuters / Landov