California Code, Penal Code - PEN § 4005

(a) Except as provided in subdivision (b), the sheriff shall receive, and keep in the county jail, any prisoner committed thereto by process or order issued under the authority of the United States, until he or she is discharged according to law, as if he or she had been committed under process issued under the authority of this state;  provision being made by the United States for the support of the prisoner.

(b) The sheriff shall receive, and keep in the county jail, any prisoner committed thereto by process or order issued under the authority of the United States, until he or she is discharged according to law, as if he or she had been committed under process issued under the authority of this state, but only if the sheriff determines that adequate space in appropriate detention areas currently exists for this purpose.  Provision shall be made by the United States for the support of the prisoner.  This subdivision shall apply only in counties where a facility operated by the United States Bureau of Prisons exists within 200 miles of the county seat.