Sprouts, organic farming and many unanswered questions

Government officials point to an organic farm as the source of German deadly E.coli despite the fact that all laboratory tests from the farm's food was negative for the E.coli. 

Epidemilogical evidence was used instead to pinpoint the specific farm as the source of the deadly bacteria from contaminated sprouts.

But the question remains, how did the farm become contaiminated in the first place.  Why isn't there any evidence of the E.coli present now?  Was it a natural contamination or was the E.coli purposely used to contaminate organic food crops?

759 people suffered serious complications from kidney failure from the outbreak.

http://news.yahoo.com/s/ap/20110610/ap_on_he_me/eu_contaminated_vegetables?om_rid=NR04UT&om_mid=_BN8nRmB8bpM5uO