How forensics can link suspect to U.S. mail bombs


AP — The pipe bombs mailed to top Democrats this past week offered all sorts of forensic clues to who had sent them.

FBI Director Christopher Wray said Cesar Sayoc’s fingerprints and possible DNA were collected from two of the 13 devices. Wray said the fingerprints matched a print found on one of the packages sent to Rep. Maxine Waters of California.

Before advances in DNA, other types of physical evidence often helped authorities. But even something as small as a stray hair can help identify a suspect.

David Chipman, a retired agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives who is now a senior adviser at the Giffords Law Center to Prevent Gun Violence, recalled working on a case in Texas in which a dog hair found on electrical tape on the device helped prove who made the bomb.

Experts said DNA or fingerprint evidence does not necessarily steer authorities directly to the perpetrator but is used to verify that the suspect they have identified is responsible.

The bombs seized were made using 6 inches (15 cm) of PVC pipe, a small clock, a battery, wiring and “energetic material.” Previously, law enforcement officials said they also contained shards of broken glass and were wrapped with black tape.

The use of broken glass and PVC pipe could point to the bomb maker wanting to ensure the devices were as light as possible to avoid shipping restrictions.

Broken glass as filler would be lighter than nails or other metal, and a PVC pipe would be lighter than a metal pipe. The postal service requires packages weighing more than 13 ounces (368 grams) to be shipped from a retail counter, and it returns heavier packages that are dropped into a mailbox or slot.

Each of the devices was packaged in a manila envelope lined with bubble wrap. Each had about six postage stamps bearing a picture of an American flag.