Officials on Friday traced the outbreak to sprouts from a farm in northern Germany but are still puzzling over how the bacteria got there — whether through workers, seeds or by other means.

The federal risk assessment agency said authorities suspect homegrown sprouts may be behind one infection in a family in northern Germany — though it says tests so far haven’t found the bacteria in the seeds.

Authorities already had issued a wider warning against eating raw sprouts. The aggressive strain of E. coli has been blamed for 36 deaths, all but one of them in Germany.