>> no weapon was found at the scene. and no shell casings either. >> to that tell you anything? that there were no shell casings? >> yes. it told us it was going to have to be a revolver. >> that's because revolvers keep bullet cases inside the gun after firing. >> i can only imagine how terrified he must of been in those final moments. >> mark was doing when he did every day. just going to work. >> now his federal prosecutors feel like he died for that work. and they could be next! no one felt safe. >> all of us were scared. we didn't know if there was a bunch of people waiting trying to shoot us. >> the courthouse was locked down in the morning, then closed during the day. prosecutors and office staff were given extra security. >> as we were walking to our cars, they were carrying huge rifles and guns. >> when did it start to sink in for you that this could be me? you walked into work with mark. your offices right there. >> i think that was the immediate fair. it could've been any of us.