Twenty civil liberties advocates and legal experts have written Attorney General Griffin B. Bell urging that he continue a Justice Department probe of FBI agents "without fear of favor."

A letter sent Tuesday noted there is growing controversy over the indictment last month of a retired New York FBI supervisor, John J. Kearney, on charges of illegal wiretapping and mail opening during an investigation in the early 1970s of the Weather Underground.

FBI Director Clarence M. Kelley, citing low morale in the bureau, has asked Bell to halt prosecution of Kearney and other FBI officials who are targets of the Justice probe. Kelley acted after some department lawyers urged Bell to prosecute middle-echelon personnel for the FBI's use of burglaries and other illegal surveillance to get information about the radical organization.

The Tuesday letter, signed by a group including former Attorney General Ramsey Clark and former Supreme Court Justice Arthur J. Goldberg, said, "We write to express our strong support for your efforts to investigate and prosecute culpable individuals."

Charging that "there appears to be a systematic effort under way to frustrate any indictments," the group asked. "What of the morale of a public that sees its law enforces become law breakers - and then be given immunity?"

The group also said that "when justice becomes politicized, it is no longer justice . . . We concur that any retrenchment in this area could be the equivalent of a green light for illegality."

Among the signers were Yeshiva Law School Chairman Mooris Abram, Harvard law professor Derrick Bell, D.C. Bar Association President Charles R. Work, Common Cause President David Cohen, and American Bar Association President Justin Stanlye.