The trial of a San Diego man who allegedly robbed his neighbors of their daughter by night and left her dead in a ditch weeks later could rest on a single fiber.

An orange acrylic fiber was abundant in defendant David Westerfield's home, but showed up only once on the victim, 7-year-old Danielle van Dam— wrapped between the links of a necklace that was still on her neck when she was found.

"The long bright orange fibers were significant to me because I had seen a bright orange fiber somewhere else and that triggered my memory," said police criminalist Jennifer Shen. "In the necklace, tangled in hair that was tangled around the necklace was a long, orange fiber."

However microscopic, the evidence delivered yet another blow to David Westerfield's defense. If convicted, Westerfield could face execution.

Prosecutors claim that the 50-year-old engineer snuck into Danielle van Dam's family home the night of Feb. 1 and took her from her bed. Her naked body was found three and a half weeks later by a roadside.

Piecing together the "trace evidence," such as fibers found in the belongings of Westerfield, has helped prosecutors link Westerfield to van Dam in the days before her murder.

The orange fiber found in van Dam's necklace, Shen testified, was everywhere in Westerfield's home. The witness told the court that it matched 20 to 30 fibers found in Westerfield's washer, 50 to 100 found on top of the washer, another 50 to 100 in his laundry, and 10 to 20 found in the bedding in his master bedroom.

Blue-gray fibers found in greater number on the victim also matched fibers found in Westerfield's home, she said, noting that the evidence was indeed more conclusive when taken as a whole.

"The fact that it is in so many places... it's certainly an additive effect," Shen said.

Westerfield and his lawyer, Steven Feldman, have claimed throughout the four-week trial that any forensic link between van Dam and Westerfield could have been established when she visited his home only days before her disappearance.

On cross-examination, Feldman pressed Shen on the precision of fiber evidence, asking her whether such trace amounts of evidence could be transferred easily from one place to another.

"Are you guessing?" he asked, singling out her link between the orange fiber and those from Westerfield's belongings.

"It's a reasonable interpretation," Shen replied.

Feldman further attacked the conclusiveness of the evidence, suggesting the fibers spotted on the body could have come from a myriad other sources, and could even have been blown by the wind onto van Dam's decomposing body.

Shen also admitted that the orange and blue-gray fibers were not the only fibers found in Westerfield's belongings, and that five total types were found in all.

But while Feldman endeavored to discredit the fiber evidence as imprecise, another prosecution witness testified that the fibers in question could not have come from one location in particular: the victim's own belongings.

According to police criminalist Melvyn Kong, who also specializes in trace evidence, none of the fibers found in van Dam's bedding and home matched the orange acrylic fiber. That the key fiber did not likely come from the most likely source —van Dam herself— could further link Westerfield to the crime in the eyes of jurors.

The meticulous process of fiber analysis first requires investigators to comb through objects with a microscope, looking for tiny out-of-place fibers that may have rubbed off or fallen from clothing, carpeting, furniture, or other types of fabric. Criminalists then use more powerful microscopes to look for traits specific to the manufacturing process, and run the fibers through a battery of chemical tests meant to identify the type of material and the dyes used to color it.

On Monday, a criminalist testified that microscopic fibers found on the floor of David Westerfield's recreational vehicle are consistent with the carpet of van Dam's bedroom.

The jury is expected to tour Westerfield's motor home later this week.