Fibers and Probability Theory
From 1979 to 1981, someone was killing Atlanta's youth. More than twenty-five black males, some as young as nine, had been strangled, bludgeoned or asphyxiated. A few females were killed and some children were just missing, but all potential leads turned into dead ends. The only real clue---which was valuable only if a suspect surfaced---was the presence on several of the bodies and their clothing of some kind of fiber threads. A few also bore strands of what was determined to be hair from a dog.
These specimens were all sent to the Georgia State Crime Laboratory for analysis, and technicians there isolated two distinct types: a violet-colored acetate fiber and a coarse yellow-green nylon fiber with the type of tri-lobed (three branch) qualities associated with carpets. They searched unsuccessfully for the manufacturer.
The fiber discovery was reported in the newspaper and shortly thereafter, bodies were found stripped and thrown into the river. Some authorities surmised that the killer believed that the water would wash away trace evidence. They took it to mean that the killer (or killers) was paying attention to the media. (Others, however, did not think that all of these deaths were related.)
Since the unknown predator seemed to favor the Chatahoochee River, the police set up a stakeout. On May 22, 1981, this strategy appeared to pay off. In the early morning hours, the stakeout patrol heard a loud splash. Someone had just thrown something rather large into the river. On the James Jackson Parkway Bridge, they saw a white Chevrolet station wagon, and when they stopped it, they learned that the driver's name was Wayne Williams. He was a 23 year-old black photographer and music promoter. They questioned him, but when he said he'd just dumped some garbage they let him go. (Later he would claim that he'd come there to see the stakeout, having heard about it from friends in the police force.)
Only two days later, the police found what they believed had been the source of the splash---the body of 27-year-old Nathaniel Cater. He was dredged up about a mile from the bridge, and despite his murderer's carefulness, a single yellow-green carpet fiber was found in his hair. (The assumption was that it had stuck there despite the water rather than thinking that he might have acquired it in the water.) Cater also showed signs of asphyxiation, but it was difficult to determine just how this had happened. Nevertheless, the medical examiner thought that he had been dead for at least two days.
The police got a search warrant for Wayne Williams' home and car, and the search turned up a valuable piece of evidence: The floors of Williams' home were covered with yellow-green carpeting, and he also had a dog. Comparisons from the samples removed from the victims showed good consistency with Williams' carpet. Although Williams claimed to have an alibi, the description he gave of his movements the night they found him on the bridge was partly false and partly unsubstantiated. Three separate polygraph tests indicated deception on Williams' part.
Then FBI experts analyzed samples from his rugs. With special equipment, and in consultation with Du Pont, they managed to ascertain that the fibers came from a Boston-based textile company. The fiber was called Wellman 181B and it had been sold to numerous carpet companies. Each uses its own dye, so that made it possible to narrow down the likely source, which was the West Point Pepperell Corporation in Georgia. Their "Luxaire English Olive" color matched that found in Wayne William's home. There were also similarities between the hair from Williams' dog and the dog hair found on several victims.
However, many other homes had this carpeting installed, too. Thus, it had to be determined just how likely it was that Williams' carpeting was unique enough to persuade a jury of his connection to the murders. The next step was calculating the odds.
A look into company records turned up information that they had only made that type of carpet during a one-year span of time, with over 16,000 yards of carpet distributed throughout the South. In comparison with the total amount of carpet distributed across the country, this was a very small sample. That made the statistical probability of the carpet being in any one person's home to be slight, if it could be assumed that Luxaire English Olive had been fairly evenly distributed. Altogether they figured that around eighty-two homes in Georgia were carpeted with Luxaire English Olive. That meant the odds were stacked against finding many homes in Atlanta: 1 in 7792.
To make their case, the prosecution relied on only two of the twenty-eight suspected murders---the one from the river, Nathaniel Cater, and another recovered in the same general area a month before, Jimmy Ray Payne (although it had not been concluded that he had been murdered). A single rayon fiber had been found on his shorts, which was consistent with the carpeting in Williams' station wagon. In this second case, statistical probability was also employed. With Chevrolet's help, the investigators determined that there was a 1 in 3,828 chance that Payne had acquired the fiber via random contact with a car that had this carpeting installed.
When the odds in both cases were multiplied, the law of probability that both men could have picked up these fibers in places other than Williams' home and car came out to 1 in almost 30,000,000. That seemed pretty staggering.
The prosecution also introduced into evidence the fibers found on the bodies of ten of the other victims (allowed in Georgia courts), which also matched those in Williams' car or home. These, they claimed, showed a pattern, and taken altogether, it increased the odds in the fiber evidence into numbers that no one could even comprehend. In total, there were 28 fiber types linked to Williams. In addition, several witnesses had come forward to place Williams with some of the victims, and others claimed to have seen suspicious scratches on Williams' arms.
After only twelve hours, the jury returned a guilty verdict, with two life sentences. The police announced that twenty-two of the unsolved murder cases were now closed, despite the fact that there was no real proof for those victims.
Subsequently the Williams conviction has become controversial. To understand this, let's look at how fiber analysis is done.