BERLIN - German authorities said Sunday that 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 35 deaths.

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

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

Tests on about 1,100 samples, nearly 300 of them from the farm, are ongoing in an effort to answer that question, the ministry said, but they have produced no positive results yet.

The total number of deaths linked to the outbreak now stands at 35, according to the World Health Organization - all in Germany, except for one in Sweden.

Twenty-three of those who died had developed a rare complication that can lead to kidney failure. In all, 3,255 people have been taken ill.

German Health Minister Daniel Bahr has cautioned that, although the outbreak appears to be abating, further deaths are still possible.