every single shooting scene inside of that building to reconstruct exactly what happened yesterday morning. >> this was a methodical and time-intensive process that includes bullet trajectory and crime scene mapping. and with the assistance of our evidence response personnel from our baltimore and richmond field offices, we will remain there for as long as necessary to carefully process each shooting site. >> the fbi says aaron alexis entered that building with a single weapon, a shotgun. federal sources tell fox news, he bought that shotgun on saturday and took it to a shooting range over the weekend before bringing it here. and bill, we're told the other weapon he used was a handgun that he acquired from one of the officers that he was involved in a shootout with. those two weapons, we're told, are the ones he used. >> the shootout, we learned, turned into a lengthy affair with police. what do you have on that, rick? >> reporter: well, metropolitan