COSBY CASE KEY WITNESS STILL SHAKEN.

Byline: Jaxon Van Derbeken Daily News Staff Writer

The Sherman Oaks woman who reported Ennis Cosby's murder remains the investigation's key witness, but police said Friday that her fragile emotional state is slowing efforts to find the killer.

The 27-year-old son of entertainment giant Bill Cosby had met the 47-year-old screenwriter at a Los Angeles party last week, and he was en route to visit her in Sherman Oaks after working out in a gym Wednesday night.

Exactly what happened in the moments before a single gunshot took Cosby's life about 1:45 a.m. Thursday atop the Sepulveda Pass remains a mystery.

Police say the woman reported finding Cosby dead next to his Mercedes-Benz, after he had called her on his cellular telephone to ask for help in changing a flat tire.

But the New York Daily News reported that she arrived before he was shot, watched him begin to fix the flat, drove away in her Jaguar when she saw a man walking toward Cosby, and then returned to find him shot to death. Los Angeles Police Department officials refused to confirm the report.

Police disclosed the existence of a possible second witness, a security guard who had been patrolling the tony Bel-Air neighborhood 20 minutes to 30 minutes before the murder. But they noted that his descriptions conflicted with the woman's, and their value was unclear.

Los Angeles Police Chief Willie L. Williams, during a news conference Friday, refused to provide specific information about the case, although he said the woman remains the best source of clues.

``The people of Los Angeles should always be concerned and use caution until we get this individual into custody,'' Williams said.

``We do not know whether it was an opportunistic robbery, whether it was something that was planned, whether it was a follow-up. We just don't know right now,'' Williams said.

What is known is that when police arrived at the desolate stretch of Skirball Center Drive, they found Cosby dead and the woman clad in a fur coat and high heels. She told them that at Cosby's request, she had driven to meet him, to use her Jaguar to illuminate him while he worked on his car.

Questions that Williams refused to answer included: whether anything had been stolen from Cosby, whether there was more than one witness, what exactly the lone witness had seen, whether the witness's account had changed, whether the witness was involved in the slaying and how the killer had escaped.

In interviews with police, the woman described a ``white male'' as the person she saw at the crime scene upon her arrival. But so far she has been unable to provide enough detail for a composite sketch of the suspect, stymieing efforts of the police to enlist the public in the search.

Williams said the goal is to ``calm her down and have her relaxed enough'' so that she could provide a better quality sketch.

``We are working with her and trying to make sure that when we have a sketch that's available, that it's something that is good, that it's really focused towards one individual rather than a general person,'' Williams said.

The woman reported Cosby's shooting by calling 911 on her cellular car phone, but police said they would not release the tapes.

``This is an individual who witnessed a homicide,'' Williams said. He declined to explain what part of the homicide the woman had witnessed, or whether she had just seen the suspect.

Investigators said the second possible witness's descriptions were enough to complete a composite sketch, but it conflicts with the woman's, police said.

Other tips are flowing into Parker Center, where the LAPD's best detectives are directing the investigation.

``We have been contacted by several people who believe they may have seen something. They may have seen Mr. Cosby before the shooting when he was still alive or they may have seen something shortly thereafter. We're in the process of contacting them and following it up,'' Williams said.

Williams pleaded with reporters not to identify the woman, even though the New York Daily News and Associated Press had reported her name. In the first hours of the investigation, images of her at the crime scene were broadcast on TV across the country.

Williams said the woman now is in seclusion and fears retaliation from whoever shot Cosby.

The night before the killing, Cosby had spent two hours working out as a guest at Sports Club LA, an exclusive Westside fitness center, leaving at 10 p.m., employees at the club said.

Cosby was in Los Angeles on vacation from Columbia University Teachers College, where he was pursuing doctoral studies. He was due to return to New York City on Tuesday, in time for classes to resume.

The green Mercedes he was driving was leased to Cosby Enterprises.

Investigators have sent the flat tire to a lab for analysis to determine whether someone sabotaged or shot the tire or whether the flat was routine, a source close the investigation said.

Further, detectives are poring over records of similar incidents, including robberies and shootings along the freeway, to see whether there are any suspect descriptions from those cases, a source said.

Los Angeles Mayor Richard Riordan, who himself lost a son in a drowning accident, sent a letter of condolence to the Cosbys on Friday, promising them that the LAPD will do what it can to catch the killer.

``As a father who has lost his only son to tragedy, my heart goes out to you and your family,'' Riordan wrote. ``I know that the pain of this loss will be with you always, but will be eased by the support and strength you provide to each other.''

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Photo

Photo: Flowers lie on the shoulder of the street where Ennis Cosby was slain.

Michael Owen Baker/Daily News