NASHVILLE, Tenn. (NewsNation Now) — The circumstances of a downtown Nashville explosion on early Christmas morning has brought the power of the FBI and ATF to Tennessee to investigate what happened and who is responsible.

“The first that came to mind was the incredible amount of damage that was done to the areas,” said homeland security and counterterrorism expert Dr. Erroll Southers.

The circumstances leading up to the blast is one thing Southers is looking to for clues: shots fired calls and an audio warning being broadcast from the RV where the blast was believed to have come from.

“The two don’t mesh,” he said. “If you have a shots fired call, obviously officers are going to respond. But then you have this audio.”

The audio recording telling people to evacuate is a tactic from the decades ago, according to Southers.

“The IRA — the Irish Republican Army — used to employ when they were going to attack something they didn’t want to kill people, but they definitely wanted to send a message,” Southers said. “So, they would give the authorities a certain time frame to get people out.”

The other question Southers has is whether the shots people heard were actually gunshots or audio from the RV?

“But, what is good is that people were able to evacuate,” he said.

Rudabeh Shahbazi: Do you see any significance to Nashville? To the AT&T building? The area? The time in which this explosion occurred and also that it was on Christmas Day?

Southers: You give us an incredible list of options here with regards to motivation. You’ve got one of three of the largest days in America. Christmas Day, New Year’s Day, and Independence Day. So, you’ve got Christmas Day, you’ve got Nashville, that’s very interesting because the only thing I can think about is the proximity to the AT&T building, which of course is critical infrastructure. It caused massive outages for cellphones, it caused the FAA towers to slow air traffic for a period of time.

Southers said there had been the threat of an infrastructure attack in the United States within the last week, although not in the Nashville area. Nashville Metro Police Chief said it was a complete surprise and there was no indication an attack was being planned.

“Sophistication will certainly lie in the bomb,” Southers said. “Every bomb has a fingerprint and they’ll figure out from analysis and investigation what this bomb was, how it was built and possibly where it was built.”

Southers is the Director of the Safe Communities Institute at the University of Southern California, where he is also a Professor of the Practice in National & Homeland Security.

Dr. Southers is also the Managing Director for Counter-Terrorism & Infrastructure Protection at TAL Global, an international security consulting firm.

Previously, he served as deputy director in the California Office of Homeland Security under California Gov. Arnold Schwarzenegger and a former FBI special agent.

NewsNation also talked to another former FBI special agent Phil Andrew on Friday night. He is the principal and founder of PAX Group, LLC.

“In situations like this, the FBI because of their jurisdiction over terrorism,” Andrew said. “And the assumption here, anytime there is a bomb and a bomb that is directed toward people or infrastructure, that’s going to be related to some sort of international or domestic terrorism. We don’t know that now, but investigators will be looking to find any evidence of that.”

Every piece of the explosion could be part of the evidence, according to Andrew.

“The truth is, they can bring a lot of personnel in to help with those sweeps,” he said.

The clues won’t just find the suspect, but help with the prosecution according to Andrew.

The importance of little pieces of evidence have been important in the past, Dr. Southers said. He pointed to the 1993 bombing as an example, where agents found the VIN of the truck used in the World Trade Center attack.