18 U.S.C. § 3052 : US Code - Section 3052: Powers of Federal Bureau of Investigation

      The Director, Associate Director, Assistant to the Director,
    Assistant Directors, inspectors, and agents of the Federal Bureau
    of Investigation of the Department of Justice may carry firearms,
    serve warrants and subpoenas issued under the authority of the
    United States and make arrests without warrant for any offense
    against the United States committed in their presence, or for any
    felony cognizable under the laws of the United States if they have
    reasonable grounds to believe that the person to be arrested has
    committed or is committing such felony.