Science on the Case

by Courtesy of Salient Stills GOTCHA
Video enhancement let police in Milan, Italy, get a better look at the gunman in an April 2000 bank holdup. Courtesy of Salient Stills

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WHAT THE INSECTS SAY


Arthropods are often the first to encounter a corpse, so maggots are prized by forensic scientists for the valuable clues they offer. As a carcass decays, it attracts various insect species. Typically the blow flies arrive first, within minutes. Other insects prefer to wait and feast on the body's protein as it ferments. By observing which species are present and what stage of development they have attained–egg, larva, pupa, or adult–and comparing that information to local weather data, entomologists can estimate the time of death.


To promote greater accuracy, forensic entomologists in Canada are developing a national database of insect life cycles. They're collecting this information by laying fresh-killed animal carcasses in fields and noting which bugs arrive first, how long they stay, the turf wars they engage in, and the kinds of damage they wreak.


PIXEL BY PIXEL

British detectives were elated when they discovered that a closed-circuit security camera atop a London building might have captured the moment in which 13-year-old Amanda Dowler was abducted on her way home from school in March. But as they watched the tape, their hopes sank. Just as Dowler was coming into view, sun glare obscured the image. Two minutes later, the glare disappeared-and so had Dowler.


To reconstruct the 2-minute gap, London officials approached FBI forensic scientists, who have had some success recently in repairing damaged videotapes using digital image processing techniques. First the tiny dots known as pixels are converted into numbers that represent the brightness of each dot. Software then unleashes algorithms to search for "noise" and remove it or increase contrast at those spots in the tape. Or the work can be done by hand: By selectively adjusting pixels-such as those representing the glare in the foreground-details that are obscured can be revealed. As this article went to press, the FBI had been able to unearth images of two people on the tape-although not Dowler. London police sought the pair, a man and a woman, for interviews.


NOW 10 BILLION TO ONE

The accuracy of DNA testing has increased in recent years thanks to a technique called polymerase chain reaction. PCR is a chemical process that replicates specific DNA sections millions of times, allowing for better manipulation of even the smallest and most degraded samples.


To conduct a PCR analysis, lab workers distill DNA from a suspect's blood and from bodily material collected at the crime scene (a minute amount of skin, blood, hair, saliva, or semen will do). From each sample, they select 13 to 16 specific segments of DNA and amplify them with PCR. The copies of each segment are deposited in ultrathin test tubes that are attached to a computerized instrument. The instrument generates a positive charge, and the negatively charged DNA travels toward it, with larger bits of DNA moving more slowly. The computer then calculates the size of each DNA segment. The probability that the selected DNA segments from one person will register as precisely the same size as corresponding segments from an unrelated person is less than one in 10 billion.


It's also possible to check a suspect's DNA against a national DNA database of convicted offenders, known as NDIS. For now DNA analysis is lab work, but in a few years, crime scene investigators may be equipped with portable DNA microchip readers that would do the work on-site.