Germany's Frankfurt (Rhine Main) Airport consists of a civilan airport on one side of the runways, and an American Air Force Base on the other side. Most military personel arrive and depart from the military air terminal on the Air Base side, but I have arrived once on a civilian flight on the civilian terminal side and had to take a military bus to the military side of the air port.
Since the news reports have not stated exactly where the attack took place we don't know if it was on the civilian side or the Air Base side. However typically you would see military buses transporting military personel to and from the military air terminal rather than the civilian air terminal. Often times, the military buses are used to transport military personel to the local train station (Bohnhoff) in downtown Frankfurt where they board civlian trains for transportation to their units scattered all around Germany and Europe.
If this incident happened on the Air Base side, I would question how this young man was allowed access to the Air Base. My guess is that the attack occurred outside the gates of the Air Base and perhaps on the civilian air terminal, or on one of the access roads around the Airport complex.
Back in the early 1970's there were several bombings on military installations in Germany attributed to the Badder-Meinhoff gang. During that time, they bombed a military installation in Heidleburg, Germany where I was stationed, and they also bombed Rhine Main Air Base in Frankfurt where this resent attack occurred. Of course, security was tightened and I assumed has remained high in light of all that has happened around the world since then.
...Rich