Boston Marathon bombing suspect Dzhokhar Tsarnaev wrote a note describing the motivations for the April 15 attack, it was reported Thursday.

Sources tell CBS that Tsarnaev wrote the confessional note while pinned down and bleeding in the boat in the Watertown, Mass., backyard where he was captured April 19, CBS News’ John Miller reported.

The note, hastily written on one of the walls of the boat’s cabin, declared that the Boston Marathon attack was retaliation for the U.S. wars in Afghanistan and Iraq, CBS News reports. The 19-year-old suspect is believed to have carried out the attacks with his brother Tamerlan Tsarnaev, who was killed by police.

Dzhokhar wrote that he considered the Boston victims to be “collateral damage,” comparing their suffering to what the world’s Muslim population has suffered as a result of America’s overseas military actions. “When you attack one Muslim, you attack all Muslims,” the note allegedly stated.

The younger Tsarnaev also claimed that he didn’t mourn the death of his brother, who died in the morning hours of April 19 after a shootout with police. Dzhokhar allegedly added that Tamerlane was a "martyr" in paradise — and that he would be joining him there soon, CBS News reports.

According to Miller’s sources, the wall on which the impromptu manifesto was written was riddled with bullet holes. The shots were fired by police officers after Dzhokhar rose up through the boat’s tarp, and they feared that he might have been strapped with another bomb.

On “CBS This Morning” Thursday, Miller said the note would be “certainly admissible” in the eventual trial of the surviving Tsarnaev brother. It contains information "consistent with what he told investigators while he was in custody," Miller added.

This is especially critical considering the manner in which investigators originally obtained information from Dzhokhar. The bombing suspect’s initial admissions were made "during the time he was interrogated but before he was given his Miranda warning,” Miller said. The note on the boat provides prosecutors with corroborating evidence against Tsarnaev, should a future court rule that statements made before the Miranda warning are inadmissible.

Without no group claiming of responsibility, authorities are still attempting to determine who else, if anyone, might have played a role in the Boston bombings. "The last big question remaining is going to be who else knew anything? Is it going to be the wife [of Tamerlane]? Is it someone overseas?" Miller said.

Still, Dzhokhar’s note demonstrates the intent behind the attack, if not the involvement of any other party. Miller suggests that the writing will be enough for authorities to officially link him to the bombings.