Source: Dzhokhar Tsarnaev left 'deathbed' note inside boat

Dzhokhar Tsarnaev left a "deathbed" note scrawled inside the hull of the boat where he was captured, claiming he and his older brother set off bombs at the Boston Marathon as retribution for U.S. attacks on Muslims in Iraq and Afghanistan, a law enforcement source said Thursday.

The note said the victims of the April 15 attack -- three dead and more than 260 injured -- were essentially collateral damage, the source said.

The college student, 19, referred to his brother, Tamerlan Tsarnaev, 26, who died in a shootout with police, as a martyr whom he wouldn't miss because he would join him in the afterlife, the source said.

The existence of the note was made public on the same day FBI Director Robert S. Mueller III defended the bureau's handling of a Russian warning about Tamerlan Tsarnaev in the months before the attack. Dzhokhar Tsarnaev escaped after the shootout four days after the bombings and was captured in the boat in the backyard of a Watertown, Mass., home.

Federal investigators have learned that Tamerlan Tsarnaev met with an exiled former Chechen rebel fighter in Manchester, N.H, less than a month before carrying out the Boston attack, according to the Voice of America. Police in Manchester told Voice of America that FBI agents have searched the home of the former Chechen resistance figure, Musa Khadjimuradov, and examined his computers' hard drives. Khadjimuradov said he had repeated contacts over the past several years with Tamerlan Tsarnaev.

On Thursday, Mueller told a Senate subcommittee that an FBI investigation found that Tamerlan Tsarnaev posed no terrorist threat. Mueller acknowledged that electronic notifications that Tsarnaev had left the United States in January 2012 and spent six months in Russia were not shared fully within the Joint Terrorism Task Force in Boston, according to The Washington Post.

Dzhokhar Tsarnaev, who is being held on federal terror charges in connection with the bombings, was badly wounded and thought he was dying when he scrawled his note, the source said. The note was stronger evidence against Tsarnaev than similar statements he made to the FBI in his hospital room, the source said. Those statements were made before he was advised of his rights and could be ruled inadmissible as evidence at a trial, the source said.If admitted in a trial, the note could serve as the equivalent of the confessional videos often produced by terrorists abroad, which are publicly released following an attack, the source said.

"This is as close as you get to that" in this case, the source said. "This is him taking responsibility."

The boat was seized by the FBI after the manhunt and the part of the boat wall containing the note will be removed for preservation and analysis, the source said. The wall was badly damaged by bullets fired by police in Watertown.