Subsequent to the filing of an application for naturalization, the Service shall conduct an investigation of the applicant. The investigation shall consist, at a minimum, of a review of all pertinent records, police department checks, and a neighborhood investigation in the vicinities where the applicant has resided and has been employed, or engaged in business, for at least the five years immediately preceding the filing of the application. The district director may waive the neighborhood investigation of the applicant provided for in this paragraph.