NEW YORK (AP)  -- The federal investigation into a high-end prostitution ring linked to Gov. Eliot Spitzer apparently began last year as a routine financial probe by Internal Revenue Service agents.

The investigation into the Emperors Club VIP, which first came to light last week when four people with charged with running the ring, was referred to the U.S. Attorney's office in Manhattan last fall.

Soon, prosecutors had amassed evidence including statements from a confidential source and an undercover officer, a review of more than 5,000 telephone calls and text messages, and more than 6,000 e-mails along with bank records, travel and hotel records and surveillance.

Conversations were recorded about someone identified as ``Client-9,'' including that a prostitute identified as ``Kristen'' should take a train from New York to Washington for a tryst on the night of Feb. 13, according to an affidavit.

A law enforcement source in Washington who spoke on condition of anonymity told The Associated Press that ``Client-9'' was Spitzer and that he met with ``Kristen'' in a Washington hotel room just two hours before Valentine's Day.

Wiretaps enabled government agents to listen in as the woman later told a booking agent for the ring that she had secured $4,300 in cash from her client and that she liked him.

It wasn't immediately clear whether Spitzer could face charges. Federal prosecutors have brought charges against several prostitution rings over the past two decades, but have generally not prosecuted the customers.

At some point the case was referred to the Public Corruption unit of the U.S. Attorney's office, which got involved after the IRS looked into a complaint of a potential violation of the Bank Secrecy Act, the government's main tool against money laundering.

Financial institutions are required to have anti-money laundering programs in place. Those programs help the government catch terrorist financiers, drug lords and other criminals. Financial companies also must report suspicious financial transactions to the government.

Investigators say the company, whose office is based in Brooklyn, made more than $1 million for its operators by selling the services of women whose bodies were displayed, their faces concealed, on a Web site.

A three-diamond prostitute would cost $1,000 per hour while a seven-diamond prostitute could fetch $3,100 and the highest paid $5,500 an hour.

Last Thursday, four people were arrested in the probe and charged with conspiracy to violate federal prostitution laws: Mark Brener, 62, and Cecil Suwal, 23, who live together in Cliffside Park, N.J.; Temeka Rachelle Lewis, 32, of Brooklyn; and Tanya Hollander, 36, of Rhinebeck, N.Y.

Brener and Suwal also were charged with conspiracy to launder more than $1 million in illicit proceeds. Lewis and Hollander were accused of arranging meetings between prostitutes and clients.

Assistant U.S. Attorney Dan Stein said he believed the arrests shut down the ring.

The business promised clients they could pay with a wire transfer that would show up on records as QAT Consulting to make it appear to be a business transaction.

``Client 9'' insisted on paying in cash.