Although there have been persistent claims of betrayal by an informant, the source of the information that led the authorities to raid the "Achterhuis" has never been identified. Night watchman Martin Sleegers and an unidentified police officer investigated a burglary at the premises in April 1944 and came across the bookcase concealing the secret door. Tonny Ahlers, a member of the National Socialist Movement in the Netherlands (NSB), was suspected of being the informant by Carol Ann Lee, biographer of Otto Frank. Another suspect is stockroom manager Willem van Maaren. The Annex occupants did not trust him, as he seemed inquisitive regarding people entering the stockroom after hours. He once unexpectedly asked the employees whether there had previously been a Mr. Frank at the office. Lena Hartog was suspected of being the informant by Anne Frank's biographer Melissa Müller. Several of these suspects knew one another and might have worked in collaboration. While virtually everyone connected with the betrayal was interrogated after the war, no one was definitively identified as being the informant.