Investigators also collect bomb fragments from throughout the blast area — and in horrible cases like the Boston bombings, from the victims — in order to try to reassemble the bomb. Once they have an idea of what the device looked like, they can cross-reference it with the FBI's Terrorist Explosive Device Analytical Center database, which collects all the improvised explosive devices used by terrorists abroad. Individual groups tend to have their own signature ways of making IEDs, using similar materials and designs, and the FBI may be able to match the device with a pre-existing series of devices in its database.
More-powerful bombs with softer casings tend to leave smaller fragments, while bombs with harder casings, such as pipe bombs made of steel, can leave fragments several inches long. If just the cap of the pipe bomb flies off, investigators may find the whole pipe. However, even small fragments on their own can be useful to investigators. For example, in the case of Pan Am Flight 103, a fragment of a circuit board that investigators say they found in a scorched shirt proved to be a crucial piece of evidence. Investigators may also try to determine the origins of individual components, such as by determining the model of watch used as a timer, or even the specific brand of glue or electrical tape used to hold them bomb together.
If investigators are able to collect large enough fragments, they are sometimes able to collect residual fingerprints or DNA evidence. One way to detect and preserve fingerprints is through "superglue fuming": In this technique, you heat superglue in a closed container with the piece of evidence, and the superglue gathers around the fingerprint, highlighting and preserving it. This is because superglue bonds remarkably well to the sweat and residue on human fingers (which is also why it's easy to get your fingers stuck together when handling superglue). On IEDs, however, fingerprints are usually burned off any fragments, so investigators will look for DNA. This may include "touch DNA": just touching something can leave dead skin cells behind, especially if the surface is rough.