The ATF has figured out where Brandon Hole was able to purchase the two guns he used to carry out the mass shooting at FedEx last week in which he killed eight people before killing himself.

Because the shooting investigation is still ongoing, they cannot say yet where he legally bought the guns. But they can say how they were able to figure out it.

“It can be very critical, especially in those initial stages,” said Suzanne Dabkowski of the ATF to WISH-TV. “You want to figure out how the firearm got into the hand of the person that used it. Were they the original purchaser or was there somebody that purchased a firearm for them?”

She says there is a drawn-out process on how to do that.

“It is not quite what you think when you see on CSI or something like that. The information is entered into what we call the e-trace system,” Dabkowski said.

Through that system, the ATF uses a gun’s serial number to contact the company that made it. They then follow the paper trail left behind by the gun’s number to the point of sale to the person who used it.

She said by law, gun store owners are required to keep records of the guns they’ve sold, bought, or traded for 20 years. It was those records kept by the two gun stores in central Indiana where Hole bought his guns that helped them confirm where he bought them.