BERLIN - Germany stepped up efforts on Tuesday to trace the source of a bacteria outbreak that has killed 24 amid warnings it might never be found, while the EU pledged aid for vegetable farmers hit by the scare.After following several apparently false leads, German authorities zeroed in on organic sprouts as the possible origin of the contamination with a highly virulent strain of E. coli.

But initial probes carried out on a farm growing a variety of sprouts in the northern state of Lower Saxony proved negative.

Another lead in Hamburg, the epicentre of the outbreak, had involved sprouts from the same farm found in the refrigerator of a sickened man. But that sample too failed to display any trace of the bacterial strain.

The scare has led puzzled officials to warn consumers off raw tomatoes...