I was wondering the same. Only The Guardian is reporting the alleged argument. German media accounts are quoting a police spokesman as saying that "everything took place inside the bus" although the body of one of the dead airmen was found just outside the bus, according to reports. I suppose things will become clearer as time goes on.
Link to Deutsche Welle article: http://www.dw-world.de/dw/article/0,,14886053,00.html