Dead Boston bombing suspect implicated in triple murder , official says . Another man , Sergei Todashev , attacks FBI agent with a knife , is arrested . Sergei had admitted to a role in a 2011 triple murder in Moscow . Authorities told him not to take a pending flight to Boston .
Dead Boston bombing suspect implicated in 2011 killings , official says . Another man , Ibragim Todashev , attacks FBI agent with a knife , is killed . Todashev had admitted to a role in a 2011 triple murder in Massachusetts . Authorities told him not to take a pending flight to Chechnya .