Dzhokhar Tsarnaev made remarks about the bombing in a makeshift message found in the boat where he was captured in the backyard of a Watertown, Massachusetts, home.
Tsarnaev scribbled that the Boston victims were collateral damage as Muslims have been during war and that an attack against one Muslim is an attack against all of them.
The source added that Dzhokhar told investigators mainly the same details in a bedside interrogation at a Boston hospital after his capture.
The twin blasts at the end of the Boston Marathon on April 15 killed three and wounded more than 260 others.
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