It was a case of hit-and-run. A speeding car struck and badly injured a boy on a bicycle. Instead of stopping to help, the car’s driver sped away and disappeared.
The police in the Canadian city of Sudbury, where the accident occurred, had very few clues. No one had seen the accident. How could they find the driver?
The car was the key. On the boy’s clothing was a small bit of paint, probably from the car that hit him. Scientific analysis showed that the paint might have come from a certain model of car, a Chrysler Coronet.
The police examined hundreds of Coronets, but the paint from the crime scene didn’t match any of them. Finally, officers found an old car with paint that matched perfectly. Government records showed who had owned the car, and the police caught the guilty driver.
Cars are involved in many crimes. As in the Sudbury case, the car might have hit someone or something. In other cases, cars transport criminals, weapons, stolen property, or even crime victims to or from the location of a crime.
Just as a criminal leaves behind fingerprints, hair, or footprints, crime cars leave signs saying, “I was here.”
The signs pointing to a crime car are called “trace evidence.” In addition to paint, trace evidence may be a tire track, a piece of glass, a spot of oil, a piece of metal, or even a whole part from a car.
Crime scene investigators (CSIs) take pictures of the scene and carefully collect objects, hoping that something might hold an important clue.
In one case, a car full of explosives blew up under a New York City building. The explosion caused a lot of damage and destroyed most evidence that could point to the driver. Among the twisted metal and broken glass, however, CSIs found part of a car’s frame.
It had a vehicle identification number on it. Police records matched the number with a car owned by a car-rental company in New Jersey. The police quickly arrested the man who had rented the car.
Tire tracks can also be especially useful. Every tire has an easy-to-see tread, a pattern of lines in the rubber. Even without special equipment, police can often see this pattern in mud, on grass, or even on the surface of a hard road near a crime scene
A case in the state of Minnesota is a good example. The police tried for four years to find a criminal who had set several forest fires. Then they found an old tire track, obviously from a truck, at the edge of a burned-out forest.
An officer took a picture and tried to match the tread pattern to the tires of trucks he saw around town. Finally, he found a perfect match, and the truck’s owner confessed to starting the fires.
Most trace evidence is harder to analyze. It must be sent to crime laboratories, where high-tech equipment can identify it. For example, a laboratory may analyze broken window glass from a crime scene. How does it bend light that shines into it?
What chemicals is it made of? With a report from the lab, a CSI can search a computer database for car models that not have found the exact crime car, but they will know what kinds to look for.
Databases matching car models to paint, glass, or other evidence get better every year. Still, CSIs have a tough job. Millions and millions of cars have been manufactured. Many cars have been destroyed.
Others are not on the streets, but parked in garages. Finding that one car used in a crime will therefore always require not just science, but also quite a bit of luck.