Boston Marathon bombing suspect Dzhokhar Tsarnaev left a note on the boat where he was found hiding from authorities at the end of a daylong manhunt four days after the April 15 attack.

Mr. Tsarnaev wrote that the bombings were retribution for what the US did to Muslims in Afghanistan and Iraq, and that the Boston victims were collateral damage, like Muslims are in US wars, an anonymous official told CBS.

“When you attack one Muslim, you attack all Muslims,” the note reportedly said.

The written confession is consistent with what the suspect told investigators about his motive during his interrogation, said John Miller, a CBS correspondent and former assistant FBI director, on “CBS This Morning.”

Since Tsarnaev talked to investigators before he was read his Miranda rights, his statement would not be admissible in court. But “these writings, in his handwriting, in the place where he was alone during that time are certainly statements that are admissible,” Mr. Miller said.

Federal prosecutors charged Tsarnaev April 22 with using a weapon of mass destruction and malicious destruction of property for the marathon attacks, which killed three people and wounded more than 260. He was arrested in Watertown, Mass., April 19 after the FBI identified him and his brother, Tamerlan Tsarnaev, as the main suspects in the bombings. Tamerlan died after a shootout with police the day his brother was arrested.

Dzhokhar said in the note that he did not mourn his brother because he was a martyr. Also wounded in the shootout, Dzhokhar said he would soon join his brother in paradise.

Authorities reportedly found the note written in pen along the wall of the boat, which was riddled with bullet holes. Miller of CBS said that the note was a way for Dzhokar to claim responsibility for the attack – claims that terrorist organizations usually make in the wake of such attacks.

Investigators, meanwhile, are looking into who else might have had knowledge of the brothers’ plans, focusing on Tamerlan’s widow, Katherine Russell, who may have seen something in the weeks leading up to the bombing or in the days after it.

Ms. Russell lived with Tamerlan and their 2-year-old daughter in a Cambridge, Mass., apartment where police say they found materials used to make the improvised explosive devices used in the attack – two pressure cookers filled with explosives and shrapnel.

She hired a new criminal defense attorney last week: Joshua Dratel, a New York lawyer who has previously represented terrorism suspects, joins two civil lawyers already representing Russell.

Mr. Dratel said Tuesday that his client has not been charged with any criminal wrongdoing and she has been fully cooperating with the FBI investigators.

"I don't see that changing in the foreseeable future," he said. "There's no inconsistency between that and her interests at this point."

Russell has been staying with her parents in North Kingstown, R.I., since her husband's death. Her lawyers said last month that she was shocked to learn of her husband’s involvement and that she had no knowledge of his activities.

Dratel would not provide details about Russell’s contact with the federal investigators beyond saying that she talked to them.

"It would be counterproductive for the investigation and for Katherine's interests for us to be more forthcoming at this time with any of the details," he said. "We wouldn't want to impair the investigation in any way."

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"It's a fluid situation," Dratel added. "We're not at the end of it."

• Material from the Associated Press contributed to this report.