The crater at the blast site will usually contain important physical evidence, because the blast will force components of the bomb into the ground. Samples of debris from the center of the explosion, such as metal, electrical tape, wire, parts of clocks or watches, wood, glass, concrete and soil are collected. This debris could contain particles of non-detonated explosive, explosive residue or components of the bomb that will help provide critical information about its construction. Working with explosive evidence at an outdoor scene is especially difficult because some residue will evaporate quickly. Such evidence should be quickly stored in airtight containers.
Some residue can be water-soluble and therefore easily lost to the environment.
When probing the 1996 crash of TWA Flight 800 off the coast of Long Island, which killed all 230 passengers and crew aboard, the FBI and other agencies had a very difficult time determining if an explosive was at fault. This crime scene, which required divers to retrieve debris from 100 feet below the ocean's surface, was probably the most difficult ever investigated. In this case, the FBI concluded that there was no terrorist attack and that no explosive had been involved in the crash. The evidence pointed to the ignition of fuel vapors in the plane's central gas tank. But if there had been a bombing and a water-soluble explosive had been used, the likelihood is that it would have gone undetected.