How is evidence from the site of an arson collected and analyzed?

When evaluating the scene of an expected arson, investigators look for specific types of evidence and patterns. Once a fire has been extinguished, investigators can begin the process of examining the scene for signs of arson. The focus of the investigation should hinge on finding the point of origin to see if an accelerant or ignition device can be located. In most arson cases, a petroleum product is used for the accelerant and a match is used as the ignition device. Under normal circumstances fire tends to move up and out forming a V-shaped pattern from the point of origin. This V-shape occurs due to the way in which fire burns; out to the sides and vertically. The origin would be at the bottom point on the V. Not all fires are as straightforward, however! When the fire pattern is "ribbon shaped" from the point of origin, this is considered a red flag and arson is usually suspected. This pattern is typically produced when fire follows the path of an accelerant that has been poured or sprayed in the area. Structures such as stairwells also often block the path of the fire and force it in another direction, so an arson expert is often called in to verify any inconsistencies in these cases.