18 U.S.C. § 1118

(a) Offense .— A person who, while confined in a Federal correctional institution under a sentence for a term of life imprisonment, commits the murder of another shall be punished by death or by life imprisonment. (b) Definitions .— In this section— “Federal correctional institution” means any Federal prison, Federal correctional facility, Federal community program center, or Federal halfway house. “murder” means a first degree or second degree murder (as defined in section 1111). “term of life imprisonment” means a sentence for the term of natural life, a sentence commuted to natural life, an indeterminate term of a minimum of at least fifteen years and a maximum of life, or an unexecuted sentence of death.