“Unknown suspect(s) fired multiple shots at two Duke Energy Substations in Moore County, North Carolina,” the FBI in Charlotte said in a statement Wednesday. “The substations are located approximately 10 miles apart in West End and Carthage, North Carolina.

“The damage led to a massive power outage of approximately 45,000 customers. The repair process will take days, therefore a state of emergency was declared in Moore County to provide resources to citizens who remain without power.”

While speaking with ABC News this week, Moore County Chief Sheriff Deputy Richard Maness confirmed that “multiple shell casings were recovered,” from the scene. No suspect has been identified, but Sheriff Ronnie Fields said that whomever carried out the attack “knew exactly what they were doing.”

Thomas O’Connor, a former FBI agent and senior consultant at The Soufan Group, explained the different ways police can use the shell casings in their investigation.

“There are several different things that leave marks that are identifiable to that weapon from that shell casing getting ejected,” O’Connor told Newsweek. “First of all, you can find the class of weapon…you’re going to know the brand of ammunition.

“Where you can get identification to the weapon is from that mark that the firing pin makes and then when the gun actually ejects that round, there’s ejectors in the weapon that kick that shell casing out. So when it does that, it makes marks on the casing itself, so those are all identifiable to that weapon.”

O’Connor also said that touch DNA could be found on bullet casings, which could further help investigators.

“Every time you load a magazine with bullets, you take your thumb and push down the bullet to get it past the spring so it locks,” O’Connor said.

According to O’Connor, if the gun or guns used in the crime were already disposed of, it might be harder for investigators to use the casings to track down the person responsible, but he noted that the casings found “could potentially be very important evidence.”

“There are many avenues of forensic identification which could be obtained from shell casings or bullet fragments,” he said. “There is case-breaking amounts of information that potentially could come from a proper forensic recovery of firearms evidence.”

On Thursday afternoon, Duke Energy announced that it had restored power to all customers in Moore County following the attack.

“Approximately 45,000 customers were initially affected when two substations in the county were attacked and vandalized on Saturday,” the company said in a press release. “The company continues to work with local, state and federal agencies on the investigation.”

Newsweek reached out to the Moore County Sheriff’s Office for comment.