The defense insists former police detective Mark Fuhrman is the pivotal witness in the O.J. Simpson murder trial. Believe him, and a bloody glove and a blood-stained Bronco link the celebrity defendant to the crimes. Doubt him, and the case crumbles.

The prosecution disputes that view, arguing that there is much other evidence to prove Simpson's guilt. But no one denies that the focus of the case for now has shifted from the defendant to the detective.

Even when his controversial tape-recordings with a screenwriter are left out of the equation, there are some unusual stories that raise fundamental questions about how Fuhrman, now 43 and retired, operated in the 20 years he was a Los Angeles police officer. There are contentions he was unstable, racist and boastful, sometimes to the point of lying.

Fuhrman's career was halted temporarily in 1981 when, at age 29, he claimed he was too stressed to work. He spent almost two years off active duty while pursuing a claim for a disability pension.

When he returned to work after the city denied him the pension, he was openly contemptuous of black female officers and there were tensions with black male officers as well, former co-workers say.

In 1987 Fuhrman swore he saw a knife at a black suspect's feet when another officer said it could not have been there. Earlier this year, the city paid $100,000 to settle a lawsuit over that incident, in which the suspect contended Fuhrman and another officer shot him when he was unarmed and had surrendered.

Two black detectives who worked with Fuhrman in West Los Angeles between 1984 and 1991 said they had no direct evidence he was a racist, nor did they know of any instance in which he had ever planted evidence. But they also said that working with him, and a clique of officers he was friendly with, was fraught with racial tension.

Joseph Rouzan, now a security consultant, was one of only two blacks among 40 detectives in the West Los Angeles Division in the mid-1980s. He said Fuhrman was part of a group of white officers that caused problems for young black female officers. Two of those young officers came to Rouzan for guidance, he said. Rouzan also said he noticed other patterns among Fuhrman and his friends.

"It's hard to put your finger on without trying to accuse people, but it's interesting that this group always brought in minority suspects, particularly the black suspects," Rouzan recalled. ". . . You rarely saw Caucasian suspects in their custody."

Judge Lance A. Ito will decide this week whether the jury will hear any of the tapes Fuhrman made over the past 10 years with a North Carolina screenwriter. So far, Ito's rulings suggest he will allow only matters that bear directly on Fuhrman's alleged propensity to plant evidence or on his purported racism.

Fuhrman's lawyers have not responded to repeated requests for an interview with their client. Anthony Pellicano, a private investigator hired by Fuhrman's lawyers, said Fuhrman cannot grant interviews.

"His life right now is in the toilet. He has no job, no future. People think he's a racist. He can't do anything to help himself. He's been ordered not to talk. His family and friends, he's told them not to get involved," Pellicano said in an interview. "Mark Fuhrman's life is ruined. For what? Because he found a key piece of evidence."

The son of a truck driver and carpenter, Fuhrman was born Feb. 5, 1952, in Eatonville, Wash., a small town near Mount Rainier. He told psychiatrists during his pension fight that his father was an "insensitive and irresponsible" man who "doesn't mind hurting people that are close to him." He called his mother overprotective. His parents divorced when he was seven.

Fuhrman had an aptitude for drawing and for a time wanted to go to art school. But he ultimately chose the Marines, joining in 1970 and shipping out to Vietnam as the war was winding down. His view of his five years in the Marines is paradoxical. On the one hand, he later told a psychiatrist, he liked the military because things were "black and white." But during his last six months in the military, in 1975, he "got tired of having a bunch of Mexicans and niggers that should be in prison telling {him} they weren't going to do something."

Fuhrman was recruited by the Los Angeles Police Department and graduated second in his police academy class. He then spent 15 months on patrol in Watts, a high-crime, predominately black neighborhood. Fuhrman then was transferred to East Los Angeles, where he worked for four years on "Mexican gangs." That, he told psychiatrist John Hochman, who evaluated him for the pension, was his "low point. . . . Those people disgust me and the public puts up with it." As a result, Furhman said, he was in a fight "at least every other day."

Hochman recounted how Fuhrman described outwitting internal affairs investigators probing the beatings of four suspects. "He says, You don't see, you don't remember and it didn't happen. Those are the three things you say and you stick to it.' " This incident appears to be the 1978 beating at Hollenbeck station mentioned in the tapes. In the tapes, defense lawyers say, Fuhrman discussed how police beat suspects in the shootings of two officers "until their faces turn{ed} to mush" and how "internal affairs was so inept, and it is like a joke with them how they covered it up."

Why, after Fuhrman applied for the stress disability, did the Los Angeles Police Department put him back on the street, knowing the statements he had made to Hochman and other psychiatrists?

Chief Willie L. Williams said at a news conference last week that management was not aware of the statements. But the records were public -- part of a lawsuit Fuhrman filed against the city over the pension. Not only did Fuhrman return to the force, he was promoted.

Pellicano said Fuhrman's statements in the pension matter, like his remarks to screenwriter Laura Hart McKinny, were not indicative of the real Mark Fuhrman. "At that time, he wanted to get out of gangs," Pellicano said. "Your life is in danger every moment. It was just posturing."

But Fuhrman had moved out of gang work at least a year before his pension fight. When he transferred to downtown foot patrol in 1980, Hochman wrote: "He describes his work as more slimes.' He says that he would be reckless' and he did not even care if he died. He said that if anyone resisted his arrest, they went back unconscious.' He was afraid that he would kill someone if he continued to work the streets."

Fuhrman's then-wife, his second, left him in March 1980. Fuhrman took a desk job a year later. He went to see a psychiatrist, and on Aug. 6, 1981, the doctor removed him from duty. For the following two years, Fuhrman worked out for two hours a day, went to his psychiatrist and took art classes at a city college.

Hochman ultimately concluded there was "some suggestion" that Fuhrman was "trying to feign the presence of severe psychopathology." Psychiatrist Ronald Koegler also said Fuhrman was "deliberately exaggerating his preoccupation with violence." But he added that the police officer was "narcissistic, self-indulgent and somewhat emotionally unstable."

Three other doctors declared Fuhrman unfit for further police work. His treating psychiatrist, David Gottlieb, said Fuhrman was "very seriously depressed and anxious, and in no way can he be assigned to duty in any part of the Police Department."

Patrick McKenna, the private detective who has spent the past year investigating Fuhrman for the defense and who uncovered the existence of McKinny's tapes, said he has interviewed hundreds of people who have known Fuhrman -- ex-wives, girlfriends, former Marines, high school classmates, officers who worked with him, people he arrested. The common thread among many of these people, according to McKenna, is that Fuhrman is a very angry man who not only uses racial epithets but "dominates the conversation, even if he doesn't know what your value system is."

When the city denied his pension in October 1983, Fuhrman was put to work in the West Los Angeles Division. A retired black detective supervisor in that division, who asked that his name not be used, said he had no problems with Fuhrman but that a black female officer came to him for help with Fuhrman.

"She told me that they were in roll call and the watch commander informed them they were going to be working together, and he straight out said he would not work with her in front of God and everybody," the black former detective said.

At one point during their shift, the detective said, the black officer noticed that a police-issued shotgun was no longer in its proper place inside the patrol car and had been left carelessly on the roof. "It was not her fault," the detective said, but she was frightened because she felt Fuhrman had put it there to get her in trouble. The detective advised her to report the matter, which she did.

"Every supervisor, whether they will admit it or not, knew about Fuhrman and the problems that he had," the detective said, adding that he had never heard of and did not know of any instances in which Fuhrman had planted evidence, beaten up suspects or used a racial slur.

"I don't dislike him," said the detective. "He's never done one thing to me to make me dislike him. You would like him. But there's a certain something about him, you know where you stand. You wouldn't say: Hey, let's go to lunch.' " Police Watch, a lawyer referral service for police misconduct, has five complaints on file against Fuhrman since 1988, a number that is not unusually high, according to the group's intake coordinator. Those allegations include charges of wrongful arrest, use of excessive force and harassment of a witness.

Robert Deutsch, an attorney who represented the suspect Fuhrman shot in 1987, said the shooting occurred when Fuhrman and three other officers were involved in a stakeout at an ATM machine where there had been robberies. The 18-year-old suspect later admitted he had a knife and was trying to rob someone. When the officers chased him, he threw the knife over a wall. When he stopped and tried to surrender, Fuhrman and another officer shot him 16 times in the jaw and torso.

There was no evidence that any officer planted the knife next to the suspect, but "the fact is Fuhrman said he saw it there in order to justify the shooting, when it couldn't be there and another officer said it wasn't there," Deutsch said in an interview. The case was supposed to go to trial earlier this year, but the city filed for a continuance because of Fuhrman's obligations in the Simpson trial. The court rejected that motion and settlement negotiations began, Deutsch said, resulting in the $100,000 payment to his client. CAPTION: Former detective Mark Fuhrman, in court with prosecutor Marcia Clark last March, testified that he found key evidence against Simpson.