The district court should enforce the Board's subpoena if the information sought is relevant to an investigation being conducted by the Board and is described with sufficient particularity. See NLRB v. G.H.R. Energy Corp., 707 F.2d 110, 113 (5th Cir. 1982); accord EEOC v. City of Norfolk Police Dep't, 45 F.3d 80, 82 (4th Cir. 1995) (noting that district court's review of administrative subpoena is limited to determining whether the agency is authorized to conduct the investigation, whether the agency "has complied with statutory requirements of due process," and whether the information sought is relevant). We, in turn, review the district court's decision to enforce the subpoena for abuse of discretion; we may reverse the district court's enforcement order "only in the most extraordinary of circumstances." G.H.R. Energy Corp., 707 F.2d at 113 (footnote omitted).
Four factors are considered to determine whether two distinct business entities are to be treated as a single employer for purposes of the National Labor Relations Act: (1) interrelation of operations, (2) common management, (3) centralized control of labor relations, and (4) common ownership or financial control. Radio Television Broadcast Technicians Local Union 1264 v. Broadcast Service of Mobile, Inc., 380 U.S. 255, 256, 85 S.Ct. 876, 877, 13 L.Ed.2d 789 (1965); NLRB v. G.H.R. Energy Corp., 707 F.2d 110, 113 (5th Cir. 1982); Penntech Papers, Inc. v. NLRB, supra, 706 F.2d at 25; Mastell Trailer Corp. v. NLRB, 682 F.2d 753, 754 (8th Cir. 1982). No one of these factors is controlling nor need all criteria be present; single employer status is a factual question that ultimately depends upon all the circumstances of the individual case.