( CNN ) -- Boston Marathon bombing victims were collateral damage in a strike meant as payback for U.S. wars in Muslim lands , the surviving suspect wrote in a message scribbled on the boat where he was found hiding , a law enforcement source told CNN Thursday . In the message , Dzhokhar Tsarnaev also proclaimed that an attack on one Muslim is an attack on all , and said he would not miss older brother Tamerlan Tsarnaev -- who died after a firefight with police three days after the bombing -- because he would soon be joining him , according to the source . The writing on the inside of the boat dovetails with what Dzhokhar , 19 , told investigators questioning him in a Boston hospital room shortly after his capture , the source said . Friendly fire led to officer 's shooting during Boston manhunt . CNN has previously cited U.S. officials in reporting that Dzhokhar said U.S. involvement in Afghanistan and Iraq were motivating factors behind the April 15 attack , which killed three people and wounded 275 . According to authorities , the Tsarnaev brothers fashioned explosive devices from pressure cookers and other materials and detonated them near the finish line of the race . Three days later , authorities released their images to the public as suspects in the case . Investigators believe they then killed MIT police Officer Sean Collier and hijacked a car before battling authorities in a wild firefight on a Watertown , Massachusetts , street . Nearly 24 hours later , police found Dzhokhar hiding in the boat after the owner called police to report someone was inside of it . The final hours that paralyzed Boston . Dzhokhar -- who suffered gunshot wounds to the head , neck , legs and hands -- is being held a federal Bureau of Prisons medical facility in Devens , Massachusetts . He has been charged with using a weapon of mass destruction and could face the death penalty if convicted . Tamerlan was secretly buried in a rural Virginia cemetery this month following protests from Massachusetts residents and officials against burying him in that state . Authorities have said they believe the brothers acted alone , but are investigating whether they could have learned from or been aided by terror groups , including groups overseas . Of particular interest has been Tamerlan 's 2012 trip to the semi-autonomous Russian republic of Dagestan , home to numerous Islamic militant groups that have warred against Moscow 's rule . Source : Russia withheld details about Tsarnaev . Russian authorities asked U.S. officials to investigate Tamerlan before the trip , saying they believed he was becoming increasingly involved with radical Islam . The FBI investigated , but found no evidence of extremist activity , FBI Director Robert Mueller reiterated Thursday during a Senate Appropriations Committee hearing . Mueller said Russian authorities told the FBI that Tamerlan and his mother appeared to be `` very religious '' and that Tamerlan seemed `` intent on returning and perhaps participating in jihad , in Russia . '' The FBI conducted a thorough investigation and found `` no ties to terrorism , '' Mueller said . U.S. officials learned after the bombings that Russian officials had intercepted a 2011 phone call between the suspect 's mother , living in Dagestan , and one of her sons , in which they reportedly had a vague conversation about jihad , a law enforcement official told CNN earlier . Some lawmakers , particularly Republicans , have been critical of how law enforcement , intelligence agencies and the administration of President Barack Obama handled the Russian tip . While Tamerlan and his mother were added to a terror database following the FBI investigation , Tamerlan was allowed to make his Russian trip in 2012 , returning six months later . Timeline : Boston attack , aftermath . CNN 's Michael Pearson contributed to this report .
NEW : FBI director reiterates investigation found no terror ties to Tamerlan Tsarnaev . The Boston Marathon bombing suspect was killed in a shootout with police early Friday morning after he allegedly rammed his car into the crowd at an April 2013 race. Dzhokhar Tsarnaev left a message in boat where he was found hiding . He wrote that the Boston Marathon bombing victims were collateral damage . Dzhokhar also said he expected to join his brother in death .