BERLIN German authorities say they haven't yet been able to resolve how sprouts at a farm became contaminated with an aggressive strain of E. coli that has been blamed for 33 deaths.

Officials determined Friday that sprouts grown at the farm in Lower Saxony state, in northern Germany, were the culprit in the outbreak which has sickened more than 3,000 people.

But the state's agriculture ministry said Sunday that it still isn't clear whether workers brought in the bug, or whether the bacteria got onto the farm on seeds or by some other means.

The ministry says tests on some 1,100 samples, nearly 300 of them from the farm, are ongoing in an effort to answer that question. But it says that they have produced no positive results yet.