18 U.S.C. § 1114 : US Code - Section 1114: Protection of officers and employees of the United States

      Whoever kills or attempts to kill any officer or employee of the
    United States or of any agency in any branch of the United States
    Government (including any member of the uniformed services) while
    such officer or employee is engaged in or on account of the
    performance of official duties, or any person assisting such an
    officer or employee in the performance of such duties or on account
    of that assistance, shall be punished - 
        (1) in the case of murder, as provided under section 1111;
        (2) in the case of manslaughter, as provided under section
      1112; or
        (3) in the case of attempted murder or manslaughter, as
      provided in section 1113.