The team was doubtful that things could have been that simple. As they had discovered from the Arrest Tracking Project, raids led by German SD officers such as Silberbauer were rare, so if this case was a deviation from the norm, it suggests that the call, too, might have been handled differently from the norm. If the call had been an anonymous tip, it would certainly not have gone directly to Dettmann, who was much too high up in the German organization to take a call from an unidentified informant.
The Cold Case Team tended to believe that the call was for Dettmann but was either an internal call from within the German organization, maybe from someone in his immediate vicinity such as Willy Lages or Ferdinand aus der Fünten, the head of the Zentralstelle, or, if an external call, was from another Zentralstelle, possibly in Groningen, Zwolle, or The Hague, or perhaps even from Westerbork internment camp. In any case, it was probably a call from someone Dettmann knew and trusted.
A 1964 statement made by Willy Lages, then imprisoned in Breda, seems to confirm this view:
So finally, you are asking me if it is logical, after receiving a telephone call about Jews in hiding at a certain, specifically named location, one would immediately go to that building to arrest the Jews found there. I would have to respond that this is illogical. In my opinion one would first check the validity of such information, unless the tip came from someone who was trusted by our department. If Silberbauer’s story about receiving a tip by telephone is correct and immediate action followed that same day, my conclusion would be that the person calling in this tip was known to us and that their earlier information had also been reliable.8
Obviously, if the caller was a highly placed source, many of the scenarios the team had considered so far could be dismissed. Someone such as Willem van Maaren, for instance, would not have had the clout to get an important man such as Dettmann on the telephone.
There are only two people who could know for sure who the mysterious caller was: the caller him/herself, and Dettmann, who received the call. But then the team thought it possible that when Dettmann called Abraham Kaper and told him to pick his raid team, he might also have told Kaper where he had gotten the information. Would there be some reference to the caller in Kaper’s files? And where would those files be after all this time?
Vince assured me that in his many years in law enforcement, he’s learned a lot about cops. Kaper might well have kept copies of the files he considered important in his own home. Though the team knew that finding any of Kaper’s descendants was a long shot （Kaper was executed as a collaborator in 1949） and it was even less likely that any of them would know about specific files, Pieter began the search.
According to records in the Amsterdam City Archives, the Kaper family originated from a region north of Amsterdam. Pieter found several people listed in that region’s phone book with the name Kaper and soon narrowed his search to a man who seemed to be the grandson of Abraham Kaper. However, every time he phoned, a woman answered and denied that her husband was in any way related to the infamous collaborator. It must not have been easy to be the grandson of one of the most notorious Dutch war criminals. At Kaper’s postwar trial, numerous witnesses attested to his ruthless brutality against Jews and resistance prisoners, few of whom had survived his interrogations.
After the war, the shame directed at collaborators and their families was intense. Most lost their jobs and had to go to the social services office of their municipality to apply for aid. The civil servants who vetted them would canvass their neighbors to ask if they knew whether the applicant was working on the side or otherwise in violation of regulations. The reports are all on file in the Social Services Archive of the City of Amsterdam.
In Kaper’s file, Pieter discovered that his wife was named Grietje Potman, and they had a daughter and two sons. From the reports of inspectors canvassing the neighborhood in the immediate postwar period, it was clear that most people detested Abraham Kaper but liked his wife.
Since Pieter was 90 percent sure that he’d found Kaper’s grandson, he decided simply to go to the Kaper residence and see what would happen. On a sunny day in June 2019, armed with a cream cake he’d bought along the way, he drove to Kaper’s apartment, let himself into the building as someone was leaving, and rang the doorbell.
Kaper himself opened the door. Pieter handed him the cake and said he had interesting information about his family. Along with his wife, Kaper turned out to be very friendly and, now in his eighties, talked freely about his past experiences with his grandfather, though he barely remembered him since he’d been a child during the war.