1. Willem van Maaren: Van Maaren was a former employee of Otto Frank's company and had access to the warehouse where the Frank family was hiding. He was also known to have pro-Nazi sympathies and may have been motivated to betray the Franks for ideological reasons.
2. Lena Hartog: Hartog was a secretary at Otto Frank's company who had access to the warehouse and may have inadvertently revealed its location to someone who passed the information on to the Nazis.
3. Pieter van Pels: Van Pels was a member of the Frank family's group of hiders and may have inadvertently revealed their location through carelessness or under pressure during interrogation.
4. Tonny Ahlers: Ahlers was a member of the resistance movement who had connections to the Frank family and may have inadvertently revealed their location to someone who passed the information on to the Nazis.
5. An anonymous tip: It is also possible that someone who knew about the hiding place provided an anonymous tip to the Nazis, either for personal reasons or for a reward.
Despite extensive research and investigations, the identity of the betrayer remains unknown.