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Methods of crime detection.

There are many methods of crime detection. Detection methods include the study of handwriting to find out who was the author of an incriminating document and the use of a lie detector test that indicates whether suspects are telling the truth by measuring their breathing and pulse and skin movements. Detectives can even use insects to help solve a murder case! By knowing how long it would take certain insects to break down body tissue, scientists can estimate the time of death. Insects can also be used to solve drug crimes. Insects are often found in illegal shipments of drugs. Detectives can use knowledge about where the insects come from to trace the drugs back to a particular location in the world.

One of the most important and reliable kinds of evidence that can be used in solving crimes is still the fingerprint. A person’s fingerprints are the swirled patterns of ridges and valleys in the skin on the tips of the fingers. These patterns are unique, that is, no two people have identical patterns, and the patterns do not change over time. Criminal investigation agencies all over the world have large collections of fingerprints to use in crime detection and these are now computerized to make it easier to search for matching prints. If fingerprints are found at a crime scene, they may be entered into a computer bank to search for a match with the prints of a suspected criminal

Fingerprints are made when someone touches a surface. Sweat and amino acids from the body transfer to the surface and leave an impression. Sometimes it is only a partial impression, but that can be sufficient. Many prints are invisible under normal circumstances, but the can be made visible using a variety of techniques, such as dusting powders and chemicals. The prints are then photographed and lifted with a tape. Today prints are often examined in darkness using high-powered lasers, and they can be retrieved from almost any surface – even plastic bags or human skin.

Of course, some crime scenes may contain no fingerprints. When the New York City police arrested a murder suspect in June 1998, they had no physical evidence tying him to the killing. Only days later they were able to link him to that homicide, plus two other homicides – and it all came down to a cup of coffee. The man, who had been arrested on a petty theft charge, was given coffee by detectives while they were questioning him. After the suspect left the room, detectives used the saliva he left on the cup to obtain his DNA. Testing then showed that his DNA matched not only the DNA found at the crime scene, but DNA associated with other crimes as well. Another case involved a DNA sample from a popsicle. Detectives who had been following the suspect picked up the popsicle from a trash bin. In this case, however, the suspect was found not to be connected to the crime.

DNA analysis is based on the fact that every person (except an identical twin) has certain elements in his or her DNA that are unique. A sample of DNA can be taken from a person and matched to a sample of DNA taken from a crime scene – from a drop of blood or a strand of hair, for example. New methods of DNA testing mean that it is now a much faster and cheaper process, and large banks of DNA test results can be stored in computers, just as fingerprints are. When DNA is collected from a crime scene, the computers can search for a test result that matches the sample. Now, in New York City, detectives are being instructed to pick up such discarded items as gum, tissues, Band-Aids, and, in some cases, the spit of suspects, in order to get samples of their DNA. If detectives can match the DNA to DNA recovered from the crime scene, chances for a guilty plea or conviction are higher: if there is no match, suspects can be eliminated.

The analysis of DNA is another example of how science and technology are transforming crime fighting. However, there are some difficulties with this method. While some see it as a positive technological advancement – a tool that helps police to find and charge the guilty and to free the innocent – others see it as a serious invasion of privacy, suggesting it is like a police search of a person or place without permission. United States law regards DNA like other “property” that someone has abandoned. If a suspect leaves saliva on a glass in a restaurant, or a cigarette butt on a sidewalk, or in some other public place, this is abandoned property. If someone drinks from a glass in a restaurant and then leaves the restaurant, he or she is, in legal terms, “abandoning” the DNA left on the glass.

There is also the issue that DNA can be used to reveal much information about a person’s genetic code. It show, for example, whether the person has genes that relate to particular illnesses or to particular kind of behavior. For reasons of privacy, therefore, it is important that DNA testing be strictly limited to simply identifying the person, and not used for other purposes without the person’s permission. Special legislation may be needed to protect this genetic information

Table 1. Determining DNA. The specimen amounts needed and the estimated likelihood of finding usable cells in them in order to determine a person’s DNA profile.
Specimen	Source or amount needed	Likelihood of usable cells
Blood stain	Size of a dime	More than 95%
Hair	Single hair with root and follicle Single hair without root	More than 90% Less than 20%
Saliva	On used gum or cigarette butt On used soda can or ground	50-70% Less than 50%
Mucus	On used tissue paper	Less than 50%
Skin cells	From socks, gloves, or other clothing used repeatedly From doorknob From handle of knife or pistol found at crime scene	30-60% Less than 20% Less than 10%