This sounded like one possibility:

Otto Frank's business friend, Tonny Ahlers, who helped him to continued his spice trade from the hiding place, betrayed the family (?). Tonny Ahlers was a member of the Nazi party.

http://www.kirjasto.sci.fi/annefran.htm

Just putting the question mark as it seems it is not clear who betrayed them in the end. The men who arrested the family have mentioned though that it was a woman´s voice (??) who told where they were....

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