D. A. to Shift Focus in Murder Trial to Evidence : Courts: After two weeks of trying to establish a motive in the death of a Westlake nurse, lawyers will pursue new testimony.

The murder of Westlake nurse Kellie O’Sullivan is the key issue in the death-penalty trial of Mark Scott Thornton, lawyers say. But in the first two weeks of the trial, they presented little testimony about his alleged link to the crime.

Beginning today, prosecutors say that will change.

Comfortable that a motive for the killing has been firmly established, prosecutors say they will now focus more on how O’Sullivan was murdered and who would have had reason to do it.

With no direct proof linking Thornton to the killing--such as a witness--prosecutors will try to use circumstantial evidence to convince the jury in Ventura County Superior Court that the 20-year-old defendant is guilty of first-degree murder and a special circumstance that could send him to the gas chamber.

Witnesses this week are expected to tell the panel that Thornton frequently hung out near the murder scene and that the murder weapon was found in his possession. Jurors have already heard that Thornton was caught driving O’Sullivan’s vehicle.

Erika Schroeder, a former girlfriend of Thornton’s, is expected to take the witness stand today and testify that Thornton had a fascination with the Santa Monica Mountains, where O’Sullivan’s body was found.

She will testify that the defendant once boasted about being able to hide a body there in a place where no one would ever find it, prosecutors said.

Paul Dougherty, a ballistics expert, is scheduled to tell the jury that the bullets used to kill the 33-year-old mother came from the same .38-caliber revolver taken from Thornton when he was arrested five days after the killing.

And Dr. James Ribe, a coroner’s official from Los Angeles, is expected to give testimony on the results of his autopsy--showing that O’Sullivan died shortly after a motorist saw two people fitting the descriptions of O’Sullivan and Thornton traveling in the direction of the mountains.

In his opening statement, Deputy Dist. Atty. Peter D. Kossoris stressed the importance of circumstantial evidence to the jury because there are no witnesses to the murder.

Up to this point, however, none of that evidence has been presented to the panel.

The prosecution has spent much of its time trying to prove that Thornton had a motive to kill O’Sullivan. That motive: He needed her vehicle.

Prosecutors say Thornton was a deeply bothered teen-ager during the time leading to the murder and decided to rob O’Sullivan of her sporty black Explorer so that he could leave the area.

Several witnesses have testified that Thornton was homeless before the murder, having been kicked out of both his home and a room he was renting.

Police had also issued an arrest warrant after he violated his juvenile probation, which he had received for a car burglary.

What’s more, his latest romantic fling was on the rocks, according to testimony.

Prosecutors have used that backdrop to support their theory that the defendant was looking to steal a car and take off for Oregon, a state he viewed as both beautiful and much safer from possible police interest in locking him up.

It was during Thornton’s hunt for a vehicle, prosecutors say, that he encountered O’Sullivan outside a Thousand Oaks pet store, on her way home after getting off work less than two hours earlier.

Prosecutors have not said whether they believe that Thornton hid inside O’Sullivan’s Ford Explorer and surprised her or if he confronted her outside it. Nor have they said whether the nurse was forced inside after leaving the pet store, where she had gone to pick up feed and a toy for her new bird.

But they have presented a witness who said she saw someone matching the defendant and the victim heading in the general direction of where O’Sullivan’s body was found.

They have had another witness testify to hearing three rapid gunshots in the mountain canyon around the time the pathologist is expected to say that O’Sullivan was shot. That witness, Donna DesBaillets, lives about a quarter of a mile from where O’Sullivan’s body was discovered.

“It was very, very loud,” said DesBaillets, adding that the shots scared her three dogs. “It just filled up the whole canyon.”

Now, prosecutors will try to prove that those were the shots that killed O’Sullivan and that only Thornton could have fired them.

Defense attorneys, who refute the prosecution’s theory, say they will call 20 witnesses to prove that their client is not guilty of first-degree murder.

They have neither conceded nor denied that Thornton was involved in the nurse’s death.

It is not clear if Thornton will be one of the defense witnesses. When asked if her client will take the stand, Deputy Public Defender Susan R. Olson laughed and said, “No comment.”

Prosecutors also still plan to put a number of police officers on the stand, but Kossoris would not say whether they would testify this week. The officers are from various agencies, including the Ventura County Sheriff’s Department and the Reno Police Department. Thornton was arrested in Reno five days after O’Sullivan’s death and gave officers there a statement.

After the prosecution witnesses testify, the jury will take a field trip to the pet store where O’Sullivan was last seen and the thick brush off Mulholland Drive where her body was found.

Jurors have already had a glimpse of the murder site. Last week, prosecutors showed them a photograph taken of O’Sullivan’s body 12 days after her disappearance, after searchers found it on Sept. 26, 1993.

Thornton burst into tears when the picture was hung in front of the courtroom, Olson said.

“I could see his eyes were getting red and his nose was getting red, and the next time I looked, tears were sliding down his cheek,” said Olson, who quickly handed Thornton a tissue.

But the trial has not always been so tense.

The jury broke into uproarious laughter Thursday when Margaret Spalding, the witness who saw the man and woman who looked like Thornton and O’Sullivan in a truck, was asked about her driving habits.

Spalding had told investigators that she drives her Mercedes 65 m.p.h. to 80 m.p.h. on the Ventura Freeway. Defense attorney Howard J. Asher asked her about the statement.

“Your honor, do I have to answer that?” Spalding asked the judge.

She then explained that she tries to “keep up with the traffic” unless she is in a hurry, prompting more laughter from the jury.

Asher asked if this has ever caused her to be ticketed.

“Not yet,” she responded. “But I’ve got a feeling that I will be.”