Another trick the SD detectives used was to place V-people in jails or camps to elicit information from prisoners about addresses where other Jews were hiding or valuables were stowed. In his postwar interrogation, Schaap admitted to sending a V-Man named Leopold de Jong to Westerbork to collect addresses from recent prisoner arrivals. He would become a potential suspect in the Annex betrayal.
Non-Jewish informants were also used, but they expected some form of payment for their information or, at a minimum, forgiveness for some prior infraction of the law, such as petty theft. People who were caught hiding Jews were given the chance to escape their fate by becoming V-people and supplying other addresses where Jews were hiding or their resistance contacts. One postwar trial involved a man who ran a boardinghouse on Weteringschans in Amsterdam and offered Jews safe accommodations. After collecting the rent in advance, he would contact the authorities to pick up the Jews. During his interrogation, he identified SD detective Frederick Cool as the officer who ordered him to call when new tenants arrived. He claimed that he followed Detective Cool’s orders because he feared that if he didn’t, he, too, would be arrested. So he made the calls—but only after he collected and deposited the rent money.
The men of IV B4 eventually found themselves making less money as the number of Jews easily found in hiding started to dwindle. Fewer arrests meant fewer bounty payments at the end of the month. To compensate for that, the Jew hunters would often steal whatever valuables the Jews or hiders possessed. For example, Dutch police officer Pieter Schaap from the Bureau of Jewish Affairs was not immune to helping himself to some of the loot. When the postwar arrest team went to pick up Schaap, they found furs, paintings, jewelry, and other valuables that had somehow made their way into the Schaap household. Witnesses claimed to have often seen Schaap’s wife wearing furs and jewelry while out on the town.
Vince was intrigued to see the name of the Dutch detective Willem Grootendorst in one of the NIOD documents. His name appeared on a number of the Kopgeld receipts, and, of course, he was one of the Dutch policemen who accompanied Silberbauer in the raid on the Annex. The other policeman on the raid, Gezinus Gringhuis, was reported for extorting a Dutch woman who was harboring a Jew. He demanded 500 guilders to “make things go away.” Instead, she went to the local police to report him. When Gringhuis went back to her house to collect his 500 guilders, police officer Hendrik Blonk was waiting behind the curtains to apprehend him. Unfortunately, a gust of wind blew the curtains open, exposing Blonk and ruining the sting. When Blonk went to Sergeant Kaper to report Gringhuis, Kaper told Blonk in the presence of his men that if he ever interfered in their business again, he would give his men the right to shoot him.3 Such was the extent of the corruption within the force.
In order to know which SD detectives worked together, Vince established the Arrest Tracking Project, an investigative initiative by which the Cold Case Team researched all arrests of Jews in 1943 and 1944 in order to determine the MOs of the Jew hunters: who worked with whom, what methods they used, how they obtained information, and so forth.
When they catalogued the information in the CABR files with Amsterdam police daily journals, Kopgeld receipts, and other sources, the group was able to see that the Annex raid was unusual in at least one regard. That day was the first time SD man Silberbauer and Dutch SD Detectives Gringhuis and Grootendorst worked together. （There was evidence of Silberbauer and Grootendorst participating in prior arrests as late as June 1944, but not with Gringhuis.） Though no one has ever categorically determined the identity of the third SD detective who joined Silberbauer and the others that day, Vince and his team came to believe that it might have been none other than Pieter Schaap. Thanks to Nienke Filius, a brilliant young Dutch data scientist who wrote a program to analyze the data from the Arrest Tracking Project, the team learned that Schaap and Silberbauer worked together in August 1944.
Often the CABR files contained mitigating statements regarding the IV B4 men’s wartime conduct. Usually the reports about “good behavior” began in late 1943, when even the SD likely suspected that the Germans would lose the war. Driven by self-preservation—everyone knew that there would be postwar reckonings—the Dutch SD detectives occasionally started helping people and letting suspects go. After the end of the war, notes flew out of the prisons asking Jews and others who’d been spared to put in a good word. In his file, Eduard Moesbergen claimed that he’d helped a prominent Jewish Council member by going to his home to warn him of an impending arrest. The council member, who was a notary by profession, was not there. Checking back a few days later and seeing that the notary was still absent, Moesbergen assumed that he’d gone into hiding.4 Of course, most of the people the IV B4 members did not help couldn’t be contacted and couldn’t testify one way or the other about their experiences at the hands of the Jew hunters; they hadn’t survived the war.