Fresh on the heels of last week's Lani Guinier fiasco, Attorney General Janet Reno is facing another potential headache on the civil rights front: A proposed landmark settlement to resolve claims of race discrimination in the FBI has come under legal attack from white agents, threatening renewed strife in the nation's top law enforcement agency.

The court settlement -- announced with considerable fanfare at a Justice Department news conference last January -- was designed to address what Clinton administration officials have identified as one of their most serious problems at the department: the paucity of minorities in the upper ranks of federal law enforcement.

At the FBI, the agency in charge of investigating civil rights complaints, there are no blacks in the top 18 management positions. Out of 56 FBI field offices, only one is headed by an African American.

"I think affirmative action in law enforcement -- or diversity, might be the better term -- is more important than anything else," Deputy Attorney General Philip B. Heymann said in an interview yesterday. "I think it's very important that law enforcement does not look like a foreign occupying force, and to do that you have to have diversity."

Although it had been worked out during the Bush administration, the FBI race discrimination settlement was seen by some officials as one of the department's best hopes for actually achieving that goal. Reached under the pressure of a threatened class action lawsuit on behalf of the bureau's 519 black agents, the settlement committed the FBI to promote or reassign specific numbers of blacks to a variety of supervisory and investigative positions.

More importantly, it pledged the bureau to conduct a top-to-bottom overhaul of personnel, promotion and disciplinary policies to eliminate what black agents claimed was "subjective" evaluation procedures that lend themselves to racial bias. While the review will be conducted by outside consultants, a three-member committee -- one of whom will be appointed by the black agents -- will "monitor and comment" on proposed changes as will lawyers for the black agents who filed the suit.

But the FBI Agents Association, a mostly white organization that represents more than 7,000 of the 10,000 bureau agents, recently challenged the settlement in federal court, branding it a "race conscious" agreement. After a vote last April by the group's national executive board, more than 1,400 FBI agents have contributed more than $37,000 to fight the battle in court.

"Our position is that for the FBI to make that settlement in its present form violates the equal employment rights of nonblack agents," said Stephen N. Shulman, lawyer for the agents' association.

Specifically, Shulman notes the agreement guarantees black agents a role in developing the policies while denying the same rights to white agents. "They are excluding nonblacks on the basis of race . . . and we don't think that is lawful under the civil rights act," he said.

Black agents no doubt have wondered how it can be argued that "nonblacks" are excluded from a process when the top echelon of FBI management remains white. But two weeks ago, over the objections of lawyers for the Justice Department and the black agents, U.S. District Judge Thomas F. Hogan approved a motion by the agents association to intervene in the case, a move that, at the very least, threatens to prolong the matter, according to lawyers for all sides.

David J. Shaffer, chief lawyer for the black agents, said he still believes the settlement will survive, although the recent actions "will further complicate the procedures." Bureau officials, however, clearly are worried.

After what one senior bureau official called the "long and tortured process" that leads up to the settlement, he said, "we're concerned that this could shake the foundation upon which this thing is built."

Hanging over the issue, of course, is the still unresolved fate of FBI Director William S. Sessions, who had strongly urged a settlement of the black agents' claims, one reason he won backing from civil rights groups earlier this year when he came under fire for alleged ethical abuses. After months of delay, department spokeman Carl Stern confirmed this week that deciding whether Sessions should stay on is now "on the front burner" and that Heymann is "working" on it.

But that does not necessarily mean imminent.

Heymann declined to be pinned down yesterday on when Sessions' fate will be decided, other than to say he would "surprised" if it wasn't within the next four weeks.