OSWIECIM, Poland — As one gazes out from the main watchtower at the grim desert that is the crumbling chimneys and crematories, vanished prisoners’ huts, barbed wire and ditches of Auschwitz-Birkenau, it is hard to fathom that there were corners of the Nazi realm where, collectively, more killing occurred than at Auschwitz.

Monday, the 69th anniversary of the day Soviet forces liberated Auschwitz, was observed as International Holocaust Remembrance Day. Yet a third or more of the almost six million Jews killed in the Holocaust perished not in the industrial-scale murder of the camps, but in executions at what historians call killing sites: thousands of villages, quarries, forests, wells, streets and homes that dot the map of Eastern Europe.

The vast numbers killed in what some have termed a “Holocaust by bullets” have slowly garnered greater attention in recent years as historians sift through often sketchy and incomplete records that became available after the collapse of the Soviet Union.

“People sat down and added the numbers up,” said David Silberklang, a senior historian at the International Institute for Holocaust Research at Yad Vashem, Israel’s official Holocaust memorial.