Lisa Faber, the supervisor of the N.Y.P.D. crime lab’s hair-and-fibre unit. Photograph by Gus Powell.

On the evening of March 10, 2003, two New York Police Department detectives, James V. Nemorin and Rodney J. Andrews, were shot and killed in an unmarked police car while attempting an undercover purchase of a Tec-9 assault pistol on Staten Island. The case was significant not just because two officers had died but because the man who was eventually charged with the murders, Ronell Wilson, faced the possibility of becoming the first person in more than fifty years to be executed for a crime in New York State.

The government’s theory was that Wilson, who was with an accomplice in the back seat of the car, shot the detectives during a robbery attempt. Among the evidence retrieved from the crime scene were hundreds of hairs and fibres, and prosecutors enlisted Lisa Faber, a criminalist and the supervisor of the N.Y.P.D. crime lab’s hair-and-fibre unit, to testify at Wilson’s trial, last winter. Under questioning in Brooklyn federal court, Faber said that she had compared samples of fabric from the detectives’ car with fibres found on gloves, jeans, and a baseball cap that Wilson had allegedly been wearing on the night of the crime. The prosecutor asked Faber to describe the methods and equipment she had used to make her analysis. Then she asked Faber what she had found. “My conclusion is that all of those questioned fibres could have originated from the interior of the Nissan Maxima, from the seats, and/or the backrests,” Faber said. She added that in her field “the strongest association you can say is that ‘it could have come from’ ” the source in question.

Faber’s testimony was careful and responsible—and not very significant. She could not say how common the automobile fabric that she had examined is, or how many models and brands use it. Nor could she say how likely it was that the fabric from the car would show up on Wilson’s clothes. Faber used no statistics, because there was no way to establish with any precision the probability that the fibres came from the de-tectives’ car. DNA tests had proved that blood from one of the detectives was on Wilson’s clothes, and based on this fact, as well as on testimony from his accomplice and from Faber, Wilson was convicted and sentenced to death. “Given how much evidence they had in the case, I wasn’t crucial,” Faber told me. “The prosecutors liked the idea of fibre evidence in addition to everything else. Maybe they thought the jury would like it because it was more ‘CSI’-esque.”

“CSI: Crime Scene Investigation,” the CBS television series, and its two spinoffs—“CSI: Miami” and “CSI: New York”—routinely appear near the top of the Nielsen ratings. (A recent international survey, based on ratings from 2005, concluded that “CSI: Miami” was the most popular program in the world.) In large part because of the series’ success, Faber’s profession has acquired an air of glamour, and its practitioners an aura of infallibility. “I just met with the conference of Louisiana judges, and, when I asked if ‘CSI’ had influenced their juries, every one of them raised their hands,” Carol Henderson, the director of the National Clearinghouse for Science, Technology and the Law, at Stetson University, in Florida, told me. “People are riveted by the idea that science can solve crimes.” At the Las Vegas criminalistics bureau where the original version of the show is set, the number of job applications has increased dramatically in the past few years. In the pilot for the series, which was broadcast in 2000, Gil Grissom, the star criminalist, who is played by William Petersen, solved a murder by comparing toenail clippings. “If I can match the nail in the sneaker to the suspect’s clipping . . . ” Grissom mused, then did just that. In the next episode, the Las Vegas investigators solved a crime by comparing striation marks on bullets. “We got a match,” one said. Later in the same show, Nick Stokes (George Eads) informs Grissom, his boss, “I just finished the carpet-swatch comparisons. Got a match.”

The fictional criminalists speak with a certainty that their real-life counterparts do not. “We never use the word ‘match,’ ” Faber, a thirty-eight-year-old Harvard graduate, told me. “The terminology is very important. On TV, they always like to say words like ‘match,’ but we say ‘similar,’ or ‘could have come from’ or ‘is associated with.’ ”

Virtually all the forensic-science tests depicted on “CSI”—including analyses of bite marks, blood spatter, handwriting, firearm and tool marks, and voices, as well as of hair and fibres—rely on the judgments of individual experts and cannot easily be subjected to statistical verification. Many of the tests were developed by police departments more than a hundred years ago, and for decades they have been admitted as evidence in criminal trials to help bring about convictions. In the mid-nineteen-nineties, nuclear-DNA analysis—which can link suspects to crime-scene evidence with mathematical certainty—became widely available, prompting some legal scholars to argue that older, less reliable tests, such as hair and fibre analysis, should no longer be allowed in court. In 1996, the authors of an exhaustive study of forensic hair comparisons published in the Columbia Human Rights Law Review concluded, “If the purveyors of this dubious science cannot do a better job of validating hair analysis than they have done so far, forensic hair comparison analysis should be excluded altogether from criminal trials.”

Last week, a commission on forensic science sponsored by the National Academy of Sciences held an open session in Washington at which several participants questioned the validity of hair and fibre evidence. Max Houck, the director of the Forensic Science Initiative at West Virginia University and the co-author of an important study that reviewed hair analyses by the F.B.I., was fiercely criticized by several members of the commission, including one of the co-chairs, Harry T. Edwards, a senior federal-appeals-court judge. “It sounds like there is a lot of impressionistic and subjective examination going on,” Edwards said, after Houck described the study. “Follow-up examiners repeated [the analyses] and made the same mistakes,” Edwards said. “That’s the scariest part.”

Sir Robert Peel is credited with creating the first modern police force, the bobbies, in London, in 1829, but the transformation of law enforcement, and especially forensic science, into a professional discipline was a haphazard affair. Scientists occasionally took an interest in police work, and courts sometimes accepted their testimony. Oddly, one prominent early figure in the field developed specialties in both bullets and hair, which have little in common except that both are often found at crime scenes. In 1910, Victor Balthazard, a professor of forensic medicine at the Sorbonne, published the first comprehensive study of hair, “Le Poil de l’Homme et des Animaux,” and three years later, in an influential article, he theorized that the grooves inside every gun barrel leave a unique imprint on bullets that pass through it. In the mid-twenties, Calvin Goddard, a New York doctor, began using a comparison microscope, which allows an analyst to examine two slides at the same time, to study bullets. In 1929, he analyzed bullets collected at the site of the St. Valentine’s Day massacre, in which gunmen wearing police uniforms shot and killed six members of George (Bugs) Moran’s gang and a seventh man. Goddard test-fired all eight machine guns owned by the Chicago police and found no match with the bullets used in the crime. Two years later, he examined two submachine guns retrieved from the home of Fred Burke, a sometime hit man for Al Capone, Moran’s great rival. Goddard pronounced Burke’s guns the murder weapons, and the feat so impressed local leaders that they established a crime lab, the nation’s first, and installed Goddard as its director.