As the investigation began, some of the first federal agents on the scene, Thursday were from the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF).
The ATF is in charge of identifying and tracking the the guns used in an incident.
Numbered markers indicated spots where evidence could be found in the recruitment center off Lee Highway. Shell casings were among the evidence collected, the metal jackets of bullets left behind when a gun is fired.
Steven Gerido, an ATF agent from Nashville says, they're like fingerprints.
"For us, shell casings are very significant. They tell a story, Each gun tells a story. So, if we get multiple casings and multiple scenes where multiple fire arms were used and we're able to enter that stuff into our National Integrated Ballistic Information System network and then we're able to link those firearms to the shell casings. What that does for us is it allows us again to get back to which guns fired which shots."
Meanwhile, federal investigators are tracking the shooter's links to radical Islam.
Tennessee's Governor Bill Haslam has ordered a security review of military facilities in the state, to see whether some military personnel can be armed to protect themselves.
Wednesday, August 16 2017 11:10 AM EDT2017-08-16 15:10:08 GMT
Confederate monuments in Baltimore were quietly removed and hauled away on trucks in darkness early Wednesday, days after a violent white nationalist rally in Virginia that was sparked by plans to take down a...More
Confederate monuments in Baltimore were quietly removed and hauled away on trucks in darkness early Wednesday, days after a violent white nationalist rally in Virginia that was sparked by plans to take down a similar statue there.More