A Study of Fitusiran (ALN-AT3SC) in Severe Hemophilia A and B Patients With Inhibitors

Inclusion Criteria: * Males, greater than or equal to (\>=) 12 years of age. * Severe hemophilia A or B with inhibitors. * (Severity confirmed by a central laboratory where coagulation factor VIII (FVIII) level was less than (\<)1% or factor IX (FIX) level was less than or equal to \[\<=\]2% at Screening; Inhibitors defined as inhibitor titer of \>=0.6 Bethesda units per milliliter \[BU/mL\] or as evidenced by medical records). * A minimum of 6 bleeding episodes requiring BPA treatment within the last 6 months prior to screening. * Willing and able to comply with the study requirements and to provide written informed consent and assent. Exclusion Criteria: * Known co-existing bleeding disorders other than hemophilia A or B. * Antithrombin (AT) activity \<60% at Screening. * Co-existing thrombophilic disorder. * Clinically significant liver disease. * Active hepatitis C virus infection. * HIV positive with a cluster of differentiation-4 count of \<200 cells/microliter. * History of arterial or venous thromboembolism. * Inadequate renal function. * History of multiple drug allergies or history of allergic reaction to an oligonucleotide or N-Acetylgalactosamine. * History of intolerance to SC injection(s). * Any other conditions or comorbidities that would make the participant unsuitable for enrollment or could interfere with participation in or completion of the study, per Investigator judgement.