How and by whom was the Secret Annex betrayed?
The occupants of the Secret Annex were discovered on August 4, 1944, when SS sergeant Karl Josef Silberbauer and three members of the Security Police arrested the Franks, van Daans (real name, van Pels), and Mr. Dussel (real name, Fritz Pfeffer), as well as Victor Kugler and Johannes Kleiman.
All that is ultimately known about the betrayal is that someone contacted the German Security Police to inform them that Jews were hiding in the warehouse. There is still no definitive answer as to who betrayed the occupants, although several suspects have emerged. Some believe that Mr. van Maaren, the warehouse manager who had often behaved suspiciously and set up traps around the warehouse to see if there were intruders, was responsible. Others believe it was Tonny Ahlers, a violent anti-Semite, who turned them in. Still others believe that Lena van Bladeren-Hartog, a woman who worked as a cleaner whose husband was employed at the warehouse, let this information slip.
Regardless of who actually committed the act, the results were tragic: a single phone call caused seven deaths, with all eight occupants being sent off to various concentration camps and only one—Otto Frank—surviving these horrific conditions.
You can find more information about the investigations of the betrayal of the Secret Annex at the following link from the Anne Frank House.