Probe of blast could take months
In the hours immediately following an urban explosion like the one that rocked Nashville on Friday morning, investigators are focused on two things: safety and collecting evidence.
“They’re really kind of determining how it happened, almost more so than why,” said Adam Hall, assistant professor in Biomedical Forensic Sciences at the Boston University School of Medicine.
“It’s difficult, if not impossible, to say how long it would take them to identify an individual or individuals of interest. But at the same time, whatever physical evidence is collected is likely being prioritized for analysis so at least they can determine the type of explosive that was used, because that can give some indication as to what the source may have been.”
Nashville authorities believe an explosion that occurred in downtown Nashville early Christmas morning was an “intentional act” and sparked by a vehicle.
Police came across a suspicious RV parked outside an AT&T building near Second Avenue and Commerce Street before 6 a.m., when initially responding to calls of shots fired in the area, said Metro police spokesperson Don Aaron.
The RV was playing a recording indicating it contained an explosive device and telling people to evacuate the area, police have confirmed.
Three people were hospitalized with injuries, police said.
Several people were taken to the department’s central precinct for questioning but authorities declined to give more details Friday morning.
The downtown area was sealed off with an active investigation underway, led by the Federal Bureau of Investigation with the help of Nashville, state and federal agencies.
Michael Knight, public information officer for the Nashville branch of the Bureau of Alcohol, Tobacco, Firearms and Explosives, said the public safety agencies were working together smoothly because of regular training in case of just such an emergency.
“We’re basically creating a timeline of that incident, from whenever it was first reported going forward, as well as from that incident when it was reported going backward to see what circumstances led up to it,” he said.
“From the safety of first responders and the public, to creating that timeline, that will tell us a whole variety of of information that will tell us, obviously, hopefully, the reasons and motivations.”
Safety is first priority
“More information may be coming in terms of the scale or the nature of the event itself, but in terms of more specifics about how it occurred or a future arrest, that’s probably a little ways down the line,” said Hall.
He was previously employed by the Massachusetts State Police Crime Laboratory as a forensic chemist and crime scene responder and is still involved in the field. He is not involved in the Nashville investigation.
Investigators’ first task is to make sure there are no secondary or tertiary devices in the area that could injure law enforcement or civilians, he said.
Though authorities said there are no indications of additional devices, police were going door-to-door with dogs in the downtown area to search nearby buildings Friday morning.
“They’re gonna want to ensure that this is a single event, that this wasn’t some larger-scale coordinated attack or that there were potentially other devices around the city,” Hall said.
Those sweeps could take hours or days, especially in a dense area like downtown Nashville, he said.
But for forensics experts, the focus will be on creating a perimeter wide enough to collect as much debris as possible to help determine the origin of the explosive device.
‘We have numerous teams that are working in parallel right now. And we brief public safety officials a couple times a day.
“We may have a team looking for evidence and one doing some investigative interviews, but then information will be collected and brought together as a whole, once a day or so and then we can see where we go from there,” Knight said.
“The balance is is 50/50. I know Nashville Metro and everyone wants to make sure that the public is in safe. We have so many law enforcement professionals out there working different angles in the investigation, not only onscene, but not on-scene that are doing background information.”
Forensic evidence can lead to suspect
As a former crime-scene responder, Hall described what investigators will be looking for as they create a perimeter and search for evidence to recreate the explosive device itself.
“The more pieces of the puzzle that they’re able to collect and put back together, the better sense that they’ll have of what the original puzzle looked like,” he said.
“If you get 100-piece puzzle and you only have five pieces you’re able to find and recover, you only have a little bit of a sense of what it may have looked like. But if you’re able to collect 80% or 90% of the puzzle pieces, even if you’re missing some, you still have a pretty good sense of what that picture originally looked like.”
A crime scene perimeter will be set at twice the distance of the evidence, he said. If investigators find relevant debris, whether that’s a portion of a vehicle or the explosive device itself, 500 feet away from the blast site, for example, they’ll expand the perimeter at minimum twice that distance.
“It’s easier to collapse a perimeter later than miss something now,” he said.
Investigators will be using aerial photography and GPS data to find and track any evidence, he said.
They’ll examine surveillance video, interview witnesses and try to determine a timeline of what happened before the blast.
“It could take weeks or months to process the evidence,” he said.
The materials used in the explosive may be key to determining who built it, Hall said.
Whether the device’s creator used easily-available materials or so-called military grade elements can help investigators narrow down a search, he explained, as well as whether they used “high” or “low” explosives.
If investigators find evidence of commonly-available materials, like those used in pipe bombs that can be bought at home improvement stores or the type of smokeless or black powder used in handgun or rifle ammunition, they may be searching for a different suspect than if the device’s creator used “high” explosives or hard-to-find materials.
“There will be a lot of attention focused on what’s known as the seat of the blast, the area of greatest damage,” he said. “[Investigators] tend to look for signs of heat damage or areas where high heat may have been experienced, and also look for metallic components, electronic components. Based on their training and experience, trying to sort out what might be a piece of a car as opposed to a piece of an electronic device.”
Although further technical details about the explosion in Nashville have not been released, Hall said it’s possible the device’s creator used the RV itself as part of the device, turning the vehicle into the casing for the explosive material. A more common pipe bomb is also possibly the source of the explosion, although Hall said in his experience a blast of this size is more likely from a larger device.
“Sounds like it may be more likely that explosives were placed into the vehicle or the vehicle was filled in part with explosives. As opposed to a typical six- or eight-inch kind of pipe bomb that’s placed into a vehicle which will do a lot of damage, but wouldn’t do the scale of damage that is was observed here,” he said. “It’s hard to say whether this is low or high explosives but it certainly seems like there’s quite extensive damage.”
Investigators will also be trying to determine how the device was set off.
“Like lighting a fuse, there’s any number of ways in which to remotely detonate something,” he said. “It doesn’t mean necessarily that video would capture the person in the act of doing it, because explosive devices can be initiated from a far, or there could have been a timer or something of that nature.”
While the forensics work is ongoing, investigators will also be looking at potential intelligence of individuals in the greater Nashville area that law enforcement may have been aware of but the public might not be. They will scour social media for any posts boasting about the explosion or that include useful information. But that doesn’t mean investigators will release that information anytime soon.
“It’s all up to what law enforcement is willing or able to release in terms of information about the investigation. They don’t want to suddenly compromise their own investigation, so more information may be coming in terms of the scale or the nature of the event itself but in terms of more specifics about how it occurred or a future arrest, I think that that’s probably a little ways down the line,” he said.