An outside investigator has begun interviewing University security guards in a probe of charges of discriminatory practices in the unit, according to sources in the Harvard police department.

Frnie Duarte, a white guard who has not charged harassment, confirmed last night he was interviewed yesterday at the Cronkhite Graduate Center by James A Ring, the director of investigative services at the Boston Law firm Choate, Hall & Stewart.

Duarte said Ring worked from a prepared set of questions, which included the question, "Do you have any firsthand knowledge of discrimination?"

Duarte termed the interview, which lasted more than an hour, "very professional."

Ring and Amos Hugh Scott, the head of litigation at the law firm who is assisting with the guard probe, have not returned repeated telephone calls during the past two weeks Ring has said in the past he does not comment on specific investigations.

Last spring, seven former and current guards charged they were harassed by their supervisors. In the fall, a Graduate School of Education student who worked in the University escort service also claimed harassment.

An investigation conducted last spring by the office of then General Counsel Deaniel Steiner '54 cleared the supervisors of the charges. Several guards criticized the inquiry because they were not interviewed.

General Counsel Margaret H. Marshall, who left Choate. Hall & Stewart last year to come to Harvard, has reopened the inquiry and asked Ring to question all guards who request to be interviewed. The goal of the investigation, according to a letter Marshall sent to more than 100 guards last month, is to eliminate any "perception" of discriminatory practices in the security department.

Some guards have suggested Ring and Scott are less than independent, because they are former colleagues of Marshall's. Marshall and Police Chief Paul E. Johnson were both on vacation yesterday and could not be reached for comment.

Ring spent 25 years at the Federal Bureau of Investigation. He was supervisor of the FBI's organized crime program in Boston from 1983 until he retired from the bureau in the fall of 1990.

While at the FBI, Ring master-minded several successful investigations of the Mafia in New England, including one that resulted in the indictments of members of the Patriarca crime family.

Ring worked with Ames Professor of Law Philip B. Heymann to investigate allegations that members of the New England Patriots football team sexually harassed former Boston Herald reporter Lisa Olson in 1990.

"He's a very good interviewer. He's fair and hard-headed," said Heymann last night. "I think he's very honest and very objective, and he reports accurately."

In 1991, the city of Chelsea, after accusations of corruption in its police department, hired Ring to give advice on issues of police and government integrity. In his capacity as adviser, Ring was allowed to conduct internal investigations of different parts of the city government.

"What I like about investigation," Ring told the Massachusetts Lawyers Weekly in 1991, "is finding there's a question, a puzzle, then devising a plan and going out and finding out what really occurred. You've got to have a high degree of curiosity."

Ring is a 1965 graduate of the University of Baltimore Law School, but he is not a practicing attorney.

Scott is a former assistant U.S. attorney. He successfully prosecuted former State Senator Jim Kelly in 1982 for fraud and for taking kick backs.

The two investigators have worked together in the past. Ring was the FBI agent assigned to the Kelly case.

Scott became a partner at Choate in 1986. In 1988, he defended Ahmad Modarressi, an Iranian accused of illegally buying missile parts to smuggle out of the country