BEDFORD, Pa. -- Prosecutors yesterday used a mixture of science and circumstance as they tried to prove that Joseph Clark kidnapped and murdered a young mother eight years ago.

A state trooper whose expertise is in fire investigations testified that somebody set the inside of Mr. Clark's car on fire about six hours after a big man kidnapped Holly Notestine from her front yard. Mr. Clark fit the general description of the kidnapper, according to Ms. Notestine's son, who was just 4 years old when he witnessed the abduction.

The trooper, Craig Grassmyer, also testified that Mr. Clark's mother said Mr. Clark had parked his car under a pear tree away from their house the night of the fire, behavior she found unusual.

"Joe doesn't park his car under the tree -- ever," Trooper Grassmyer quoted her as saying.

Assistant District Attorney Travis Livengood scored the two points that he wanted. One was that evidence of Ms. Notestine's presence in the car could have been wiped out by the fire. The other was that Mr. Clark, charged with arson as well as murder and kidnapping, parked away from the house to minimize the chance of damaging anything but his car in the fire.

Then a state police sergeant who specializes in "impression evidence" testified that one of the tires on Mr. Clark's 1985 Oldsmobile could not be ruled out as the source of a fresh track in Ms. Notestine's yard.

Throughout the testimony, Mr. Clark whispered and passed notes to his lawyers. He even spoke up once when his lead attorney, Thomas Crawford, struggled to hear what was being said.

Bedford County President Judge Daniel Howsare told Mr. Clark to be quiet and let his attorneys do the talking. Soon after, Mr. Crawford began a series of clashes with the judge over whether State Police Sgt. Kevin Deskiewicz really had expertise in tire markings.

Prosecutors objected more than a dozen times as Mr. Crawford tried to question Sgt. Deskiewicz about tire manufacturing and distribution. Judge Howsare sustained every prosecution objection except one.

"Can I ask him anything?" Mr. Crawford said, more as a complaint than a question.

Judge Howsare said the defense was free to ask questions, but he would rule them out of order if he thought they did nothing to shed light on Sgt. Deskiewicz's qualifications.

Mr. Crawford, in his opening statement last week, suggested that car fires are common and that the one in Mr. Clark's vehicle could have been started by a marijuana cigarette, smoked by his client.

But Trooper Grassmyer told the jury that he ruled out smoking as the cause of the fire.

"It was intentionally set by human hand," he told the 12 jurors and five alternates.

They were imported from Dauphin County because Mr. Clark's lawyers said an impartial jury could not be found in Bedford, a rural county of 50,000 where the Notestine case has been in the news since her disappearance on April 30, 2000.

Skeletal remains of the 25-year-old were found four years later, in between her house and Mr. Clark's. Assistant District Attorney Travis Livengood said she died from 17 stab wounds.

Mr. Crawford contends that she died from a blow to the head from an angle iron -- a piece of hard metal about 3 feet long. Moreover, he says the killer was not Mr. Clark but a convicted bank robber named David Lucas.

Mr. Lucas wrote a confession to the murder, but Mr. Crawford said he was unsure whether he would testify at Mr. Clark's murder trial, which continues tomorrow. Mr. Clark, 48, could face the death penalty if he is convicted.

His lawyer said Mr. Clark, an iron worker, was returning from a trip to Maryland when the kidnapping occurred. Further, Mr. Crawford said Mr. Clark never would have burned his car with his beloved tool sets still in the trunk.

But Trooper Grassmyer testified that Mr. Clark may have panicked to destroy blood and fiber evidence tying him to Ms. Notestine. The fire, he said, burned at well over 1,000 degrees and would have destroyed blood evidence, if it existed.