Police investigating a prostitution ring in Chester County relied on two customers, including the dean of Villanova Law School, to provide information that culminated last week in a no-contest plea by the man promoting the business, documents show.

Mark A. Sargent, who was appointed dean in 1987, resigned suddenly Monday, citing personal and medical reasons.

According to a report by the Pennsylvania State Police, Sargent was a customer at a Kennett Township house suspected as a site for prostitution when police raided it Nov. 25. He was not charged.

The Rev. Peter Donohue, the Villanova University president, was out of town yesterday and unavailable for comment, said Jonathan Gust, university spokesman.

Gust said that the university had accepted Sargent's resignation and that he would not be returning to the faculty. He declined comment on Sargent's involvement in the prostitution bust.

Repeated calls to Sargent's home in Devon were not picked up, and no one answered the door at his two-story, stucco house yesterday evening. His attorney, Bruce Castor, did not return several calls and e-mails seeking comment.

The November raid came after a six-week surveillance of the house on Round Hill Road. The dwelling's owner, Stephen A. Clark, was arrested Nov. 26 and pleaded no contest to charges of promoting prostitution and conspiracy. He was sentenced to 5 to 23 months in jail.

In an interview at the jail Wednesday, Clark, 62, of Kennett Township, said he believed a "celebrity perp," as he called Sargent, got a free pass and influenced the outcome of his case.

"That's absolutely not true," said Chester County District Attorney Joseph W. Carroll.

Customers are not charged or identified in prostitution busts, he said. Instead, he said, authorities use them to build their case and they often testify if the case goes to trial.

Carroll, who did not release Sargent's name, said he was not contacted by anyone from Villanova, where he attended law school. Carroll said he did receive a phone call Monday from Castor, who is also a Montgomery County commissioner, on Sargent's behalf.

Carroll said Castor, a former Montgomery County district attorney, wanted to know whether Carroll intended to release Sargent's name.

"I told him no," Carroll said, adding that his office rarely releases the names of people not charged with a crime.

Carroll said getting convictions in prostitution cases is always a challenge.

"Unless we have someone on the inside, there's no way to prove that sex was exchanged for money," he said. He added that Sargent and another customer in the Kennett case "were treated the same, and both would have testified if the cases had gone to trial."

For his part, Clark said that authorities did everything they could to ensure that the testimony would not be necessary. He said that after speaking with his attorney, Thomas H. Ramsay, he concluded that he could not "afford to roll the dice at age 62" and risk greater punishment, so he accepted a plea bargain.

He said police repeatedly referred to him as the "brothel operator," while treating Sargent "with deference."

"If you watch the taped interview, the police are almost apologetic with this guy," Clark said of Sargent. "They told him, 'You just happened to be in the wrong place at the wrong time,' and they agreed to contact him at his office, not his home."

Clark's codefendants - Cara Martin, 32, and Lacy Welsh, 20 - also have been prosecuted in the case.

Welsh pleaded guilty to promoting prostitution and conspiracy, and is awaiting sentencing. Martin, who has a history of arrests on prostitution and other charges in Pennsylvania, New Jersey, and Delaware, pleaded guilty to promoting prostitution and received an 8-to-23-month jail term.

Sargent paid Martin $170 for 35 minutes of sexual contact between noon and 1 p.m. on Nov. 25, according to the police report. Sargent said he saw an ad on Craigslist, "got curious," and responded to it, the report said.

The Villanova Law School's online biography of Sargent says he served as law school dean since 1997. Before that, Sargent was a professor and associate dean at University of Maryland School of Law. He is a graduate of Cornell Law School.

Sargent, who is married and has a grown son, also taught at the law schools of American University, Southern Methodist University, and the University of Baltimore. His current research and teaching interests are principally in the area of Catholic social thought and the law, the Web site said.

In 2004, Sargent authored an article in the Villanova Law Review titled "Lawyers in the Moral Maze," which focuses on why corporate lawyers often engage in behavior that violates their own ethics.

Contact staff writer Kathleen Brady Shea at 610-696-3815 or kbrady@phillynews.com.