The Justice Department used some of its most intrusive tactics against Eliot Spitzer, examining his financial records, eavesdropping on his phone calls and tailing him during its criminal investigation of the Emperor's Club prostitution ring. The scale and intensity of the investigation of Mr. Spitzer, then the governor of New York, seemed ... to be a departure for the Justice Department, which aggressively investigates allegations of wrongdoing by public officials, but almost never investigates people who pay prostitutes for sex. A review of recent federal cases shows that federal prosecutors go sparingly after owners and operators of prostitution enterprises, and usually only when millions of dollars are involved or there are aggravating circumstances, like human trafficking or child exploitation. The focus on Mr. Spitzer was so intense that the F.B.I. used surveillance teams to follow both him and the prostitute in Washington in February. Stakeouts and surveillance are labor-intensive and often involve teams of a dozen or more agents and non-agent specialists. An affidavit filed in the prostitution case did not identify Mr. Spitzer by name, only as Client 9, but it provided far more detail, some of it unusually explicit, about Client 9's encounter with the prostitute than about any of the nine other clients identified by number in the document. Several current and former federal prosecutors and prominent defense lawyers who reviewed the document said the inclusion of such salacious details about Mr. Spitzer's encounter with the prostitute went far beyond what was necessary to provide probable cause for the arrests and for searches, the purpose of the affidavit. The government has not accused Mr. Spitzer, a Democrat, of any wrongdoing.