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Inclusion Criteria:

* Males, \>=12 years of age.
* Severe hemophilia A or B (as evidenced by a central laboratory measurement at screening or documented medical record evidence of FVIII less than (\<) 1 percent (%) or FIX level less than or equal to (\<=) 2%).
* A minimum of 2 bleeding episodes required BPA treatment within the last 6 months prior to screening for participants with inhibitory antibodies to factor VIII or factor IX (Cohort A). A minimum of 1 bleeding episode required factor treatment within the last 12 months prior to screening for participants without inhibitory antibodies to factor VIII or factor IX (Cohort B).
* Met either the definition of inhibitor or non-inhibitor participant as below:
* Inhibitor: Use of BPAs for prophylaxis and for any bleeding episodes for at least the last 6 months prior to screening, and met one of the following Nijmegen-modified Bethesda assay results criteria:
* Inhibitor titer of \>=0.6 Bethesda Unit per milliliter (BU/mL) at screening, or
* Inhibitor titer of \<0.6 BU/mL at screening with medical record evidence of 2 consecutive titers \>=0.6 BU/mL, or
* Inhibitor titer of \<0.6 BU/mL at screening with medical record evidence of anamnestic response
* The subgroup of participants in Cohort A participants might additionally meet the following criteria to be eligible to start treatment with fitusiran directly after the screening period:

  * Hemophilia B with inhibitory antibody to Factor IX as defined above
  * Not responding adequately to BPA treatment (historical ABR \>=20) prior to enrollment
  * In the opinion of the Investigator, with approval of Sponsor Medical Monitor, 6-month BPA prophylaxis period should be omitted.
* Non-inhibitor: Use of factor concentrates for prophylaxis and for any bleeding episodes for at least the last 6 months prior to screening, and met each of the following criterion:
* Nijmegen-modified Bethesda assay inhibitor titer of \<0.6 BU/mL at screening and
* No use of BPAs to treat bleeding episodes for at least the last 6 months prior to screening and
* No history of immune tolerance induction therapy within the past 3 years prior to screening.
* Documented prophylactic treatment with factor concentrates or BPAs for the treatment of hemophilia A or B for at least 6 months prior to screening.
* Adherent to the prescribed prophylactic therapy for at least 6 months prior to screening per Investigator assessment.
* Willed and complied 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.
* AT activity \<60% at screening.
* Co-existing thrombophilic disorder.
* Clinically significant liver disease.
* Active Hepatitis C virus infection.
* Acute or chronic Hepatitis B virus infection.
* HIV positive with a CD4 count of \<200 cells per 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 (GalNAc).
* History of intolerance to subcutaneous injection(s).
* Any other conditions or comorbidities that made the participant unsuitable for enrollment or could interfere with participation in or completion of the study, per Investigator judgment.

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