BERLIN, Aug. 25 -- He took the bus. On the first of October last year a 24-year-old man travelled from Poland to Berlin, walked to a roadside in the east of the city, collected two parcels, carried them out to woodland near a lake in the southeast, buried them, photographed the spot, sent the coordinates to whoever had asked for them, and got a bus back to Poland. German investigators believe he knew very little more about the job than that.

Der Spiegel identified him on Tuesday as Vadym S., a Ukrainian national who had been living in Poland and who is now in pre-trial detention in Romania. He is the first person publicly connected to a weapons cache that German counter-intelligence believes was stocked on the orders of Russian intellige...