The reports led prosecutors and investigators to what some describe as a kind of crossroads. While they do not routinely investigate allegations concerning public officials who pay for sex, the Justice Department and Federal Bureau of Investigation regard public corruption as a high priority and routinely investigate allegations of even low-level wrongdoing.

“If the government gets a Suspicious Activity Report about a high-ranking public official, they would be negligent not to pursue it, if only to determine whether there was bribery or extortion involved,” said Robert D. Luskin, a defense lawyer and former federal prosecutor. Mr. Luskin said that as the case proceeds, the more difficult questions could well involve how the information about Mr. Spitzer became public and whether the government “will prosecute Spitzer if it doesn’t prosecute others in the same situation.”

The Treasury reports led federal investigators to the Emperor’s Club and Mr. Spitzer’s involvement with prostitutes, and later to the 47-page affidavit filed with the complaint that referred to him as Client 9.

The officials said that once they learned that such a prominent figure was involved in soliciting prostitutes, and had seemed to be arranging sex in violation of the statute that prohibits travel across state lines to engage in sex, they wanted to follow the evidence.

Moreover, several asserted that had they dropped the matter or given Mr. Spitzer’s conduct only cursory examination, they almost certainly would have been accused of a cover-up for failing to aggressively investigate possible misconduct by a public official.

In defending their handling of the case, officials said that in the end, investigators chose to monitor his conduct but made no effort to set up a sting, or an arranged situation in which Mr. Spitzer might implicate himself. They did not surreptitiously record his activities inside the hotel or seek to obtain DNA evidence. It was not necessary, as Mr. Spitzer proved to be easy prey, according to the affidavit, which was signed by an F.B.I. agent.

It indicated that on Feb. 13 federal agents staked out his hotel in Washington, and it contained recorded conversations that amply demonstrated that he willingly had a sexual encounter with a prostitute. Afterward she was recorded on a wiretap telling an Emperor’s Club employee: “I don’t think he’s difficult. I mean it’s kind of like, whatever.”