Serge Klarsfeld, known along with his wife, Beate, for documenting details of the Holocaust and for racking down former Nazis, maintains that 3,000 Jews were stopped from entering Switzerland.

Klarsfeld’s claim, recorded in an interview with German-language weekly newspaper Der Sonntag, contrasts with the previous estimate of 25,300 made in a report issued in 1999 by the Bergier Commission.

“Since 1999, we have made progress with our research,” Klarsfeld, 77, told the newspaper....