After an explosion, crime scene analysts diagram the blast radius, chemists take swabs of chemical residue, dogs sniff the perimeter, bomb technicians analyze bomb fragments, and old-fashioned police detectives scribble details in their notebooks that might help them with the scores of interviews to come.
“It’s kind of like an orchestra where all these things are going on at the same time, and the person in charge is making sure they’re working together,” said Mike Bouchard, a former assistant director of the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives, or ATF.
When a serial bomber is still at large, as is the case in Austin following three explosions in the last two weeks, police are usually tight-lipped about the details of their investigation to prevent perpetrators from knowing what the authorities have learned about their techniques. But explosives experts not working on the case described a fast-moving but detail-oriented approach.
The first step is to secure the scene and check for secondary or unexploded devices. Next, technicians will do field tests of substances and fragments at the scene to produce preliminary findings that may be immediately helpful to investigators. Then they will send the samples to the ATF’s laboratory in Beltsville, Md., for confirmation and further analysis.
“We look for anything that would help give us some type of insight into the bomber, which would include the type of container used, the type of explosives used, how it was initiated, and then when you have multiple ones, we’re looking for similarities,” said Tina Sherrow, who retired from ATF last year as a senior special agent and explosives specialist. “It provides a signature.”
There are three ways to detonate an explosive: a timer, a remote or an action. The Austin bomber’s packages so far appear to be action-based, specifically a “victim-operated improvised explosive device” that is “designed in such a way that it would cause the victim to take an action to it,” Sherrow said.
Such a trigger could be as simple as tying a string to a package’s flap that, when opened, pulls up a piece of cardboard separating two surfaces that combust upon contact. Some more complicated devices are triggered by movement.
Police have said the Austin bomber was “sophisticated” and put the devices in cardboard boxes that look like mailed packages but were not dropped off by delivery services. ABC News reported Tuesday that they were motion-triggered and packed with nails, bolts and other material to produce shrapnel.
Despite the explosion, bombings produce a wealth of physical evidence, which is part of the reason that cases resulting in death or injury have almost always, in recent years, led to a suspect being arrested. The recovered material is checked against the ATF’s Arson and Explosives National Repository and the FBI’s Terrorist Explosive Device Analytical Center.
Crime scene evidence collected from bombings sometimes leads investigators directly to the perpetrators, by tracing component parts of the bombs or, rarely, through DNA or fingerprints. Other times, it gives prosecutors ammunition to convict suspects in court after they are arrested based on other evidence, such as video surveillance or tips collected during the investigation.
Before the Austin bombings, there were four package or mail bomb incidents that killed or injured victims in the past 18 months, according to the ATF’s most recent Explosives Incident Report. In all but one case, authorities arrested suspects.
The death toll from all domestic bombings has been falling, from 74 in 2012 to just seven in 2016.
With serial bombers, investigators also look for connections between the victims, Sherrow said.
In the Austin bombings, all of the victims are racial minorities, and two of them came from prominent African-American families that know each other. Nonetheless, those connections alone are not enough to establish a motive, Sherrow said.
“It’s way too early to assess what that might be,” she said. “It’s an extensive, prolonged period of time where interviewing is key, and the investigators that are there that are trained and highly skilled know that.”