FILE - In this June 5, 2011 file picture greenhouses at an organic farm that grows bean sprouts are photographed in the Uelzen district, northern Germany, Sunday, June 5, 2011. Investigators have determined that German-grown vegetable sprouts are the cause of the E. coli outbreak that has killed 29 people and sickened nearly 3,000, the head of Germany's national disease control center said Friday, June 10, 2011. Reinhard Burger, president of the Robert Koch Institute, said even though no tests of the sprouts from an organic farm in Lower Saxony had come back positive for the E. coli strain behind the outbreak, an investigation into the pattern of the outbreak had produced enough evidence to draw the conclusion. (AP Photo/dapd, Axel Heimken,File)

(AP) -- 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 , 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 - all in Germany, except for one in Sweden.

Twenty-three of those who died had developed a rare complication that can lead to . 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.