WASHINGTON (CNN) -- Federal agents have determined New York Gov. Eliot Spitzer used a high-priced call-girl ring at least eight times in recent months, and agents had him under surveillance twice this year, sources familiar with the investigation said Wednesday.



Sources say New York Gov. Eliot Spitzer used a call-girl service eight times in recent months.



Spitzer announced his resignation Wednesday, two days after reports of his connection to the Emperors Club VIP became public.



He has not been charged with a crime.



He told reporters Wednesday, "I cannot allow my private failings to disrupt the people's work."



The resignation will take effect Monday.  Watch Spitzer say he will resign »



The sources said the investigation began when New York's North Fork Bank notified the Treasury Department about suspicious transfers of money from Spitzer's bank accounts. That investigation led agents to the alleged organizers of the prostitution ring, four of whom were charged in a criminal complaint last week, the sources said.



A grand jury in New York is likely to hear evidence in the case soon, said Kathleen Mullin, an attorney who said she represents one of the ring's employees. Mullin would not identify her client, but said she was not the woman identified only as "Kristen" linked to Spitzer in court papers.



She said her client and other women who worked for the Emperors Club have been asked to testify before the grand jury.



Asked if her client had any encounters with Spitzer, Mullin said, "We have no information regarding the governor."



Wiretaps on suspected members of the ring, authorized in January, yielded more than 5,000 telephone calls and text messages and another 6,000-plus e-mails, according to court papers. In those intercepts, the organizers told clients how to arrange and pay for their trysts, a federal agent's affidavit states.



The affidavit identified clients by number, with Spitzer designated "Client 9," a source with knowledge of the investigation told CNN this week. Sources familiar with the investigation said federal authorities Wednesday were trying to clamp down on leaks of the investigation's details.  See a timeline of the investigation »



The affidavit states "Client 9" paid $4,300 for 2½ hours with a call girl he arranged to meet at Washington's Mayflower Hotel, with some of that a deposit on a future session. Court papers state he also paid for train tickets, cab fare, mini-bar and room service charges for Kristen -- a 5-foot-5, 105-pound brunette he arranged to meet the night of February 13.



Kristen is a 22-year-old would-be singer from New Jersey, The New York Times reported Wednesday.



The newspaper said Ashley Youmans -- now known as Ashley Alexandra Dupre -- was identified in court documents as Kristen.



Dupre has not been charged with a crime. She made a brief appearance Monday in U.S. Magistrate Court as a witness against four people charged with operating the Emperor's Club, the Times said.



In an entry on her MySpace page, Dupre says she left "a broken family" and "abuse" in 2004, eventually settling in Manhattan "to pursue my music career."



"I am all about my music, and my music is all about me," she writes on her MySpace page. "It flows from what I've been through, what I've seen and how I feel."



Dupre's mother, Carolyn Capalbo, told the Times she and her daughter were close, adding that "she obviously got involved in something much larger than her."



Spitzer, whose squeaky-clean image as a corporate corruption-buster made him a rising Democratic star, testified to the House Financial Services Committee the next day about the effect of the mortgage meltdown. He took a state plane from Buffalo, New York, to Washington and back to New York, his office confirmed Tuesday.



Sources told CNN that FBI agents had Spitzer under surveillance at the Mayflower that night -- and on an earlier occasion, on January 26, when no prostitute showed up.



His resignation is unlikely to affect decisions
What kind of transfers were suspicious? What kind of ring was Spritzer linked to? After being linked to a call girl ring, who resigned? The bank notified whom of the suspicious transfers of money?
money from Spitzer's bank accounts. high-priced call-girl New York Gov. Eliot Spitzer Treasury Department
question: What kind of transfers were suspicious?, answer: money from Spitzer's bank accounts. | question: What kind of ring was Spritzer linked to?, answer: high-priced call-girl | question: After being linked to a call girl ring, who resigned?, answer: New York Gov. Eliot Spitzer | question: The bank notified whom of the suspicious transfers of money?, answer: Treasury Department