Paul Shapiro from the Center for Advanced Holocaust Studies at the US Holocaust Memorial Museum said well over one million Jews were murdered in Eastern Europe before the Nazi concentration camps were operational.
He said the mass graves "lay forgotten, unmarked and unstudied for decades" because of Communist rule and Holocaust denial.
The event in Romania's capital – the first of its kind in a former communist country – coincided with the commemoration of 70 years since about 12,000 Jews were killed in northeastern Romania under the pro-fascist regime of dictator Marshal Ion Antonescu.
Participants at the symposium came from France, Germany, Ukraine, the US and Romania.
Last year, researchers in Romania discovered a mass grave in a forest near the town of Popricani, close to the northeastern city of Iasi where the 1941 pogrom took place. The grave contained the bodies of 36 people, including women and 12 children.
Shapiro said "at Iasi, thousands of Jews were murdered on the streets of the city by Romanian authorities and civilian collaborators, with some German participation, within sight of the non-Jewish population of the city." He added that "the killings elicited no negative reaction by the population."
Shapiro believes that the Iasi pogrom could be seen as "the signal that mass murder of Jews on the streets, in plain sight, in public, was possible, and that all Jews, not just men who might bear arms to resist, should be killed and simply thrown into mass graves, to be forgotten forever."
"Is it by chance that just three to four weeks later official Nazi policy changed to call for the murder of all Jews - men, women, children and the old?" he said.
The discovery near Iasi last year offered evidence of pogroms against Jews in the region, where official history taught that Germans were the sole perpetrators of the Holocaust.
About 280,000 Jews and 11,000 Roma, or Gypsies, were killed during the pro-fascist regime of dictator Marshal Ion Antonescu, who was Romania's prime minister from 1940 to 1944 and was executed by the communists in 1946. About 6,000 Jews live in Romania today.
During communist times, the country largely ignored the involvement of Romania's leaders in wartime crimes.
The country's role in the Holocaust and the deportation of Jews were minimized by subsequent governments after communism collapsed in 1989.
In 2004, after a dispute with Israel over comments about the Holocaust, then-President Ion Iliescu assembled an international panel led by Nobel-prize winner Elie Wiesel to investigate the Holocaust in Romania.
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