Princeton professor Jan Tomasz Gross says prosecutors in Poland asked for information to back up his assertion in 2015 article

A Polish prosecutor has questioned a Polish-American scholar, Jan Tomasz Gross, to determine if he committed the crime of publicly insulting the nation with a statement on Polish violence against Jews during the second world war.

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Gross, a professor based at Princeton University, said he was questioned for five hours on Tuesday in Katowice but does not yet know if he will be charged with the alleged offence, which can carry a prison term of up to three years.

Poland’s case against Gross, which also involves a presidential threat to strip him of a state honour, has raised questions about the conservative leadership’s commitment to the freedom of scholarship. The ruling Law and Justice party that controls both the presidency and parliament is also centralising power in a way that has raised concerns about its commitment to democracy more broadly.

Gross was questioned after multiple complaints were filed with prosecutors by Polish citizens over an article published in 2015 in which Gross said Poles killed more Jews than Germans during the German occupation – a claim that challenges a widespread conviction in Poland that the Polish response was almost exclusively honourable.

Gross made the comparison in an article published by Project Syndicate in September critical of how Poland and other eastern European countries have reacted to the migrant crisis. He decried as “heartless” the region’s opposition to accepting refugees and argued that the attitude was rooted in the region’s “murderous past”.

In the most controversial section, Gross wrote: “Consider the Poles, who, deservedly proud of their society’s anti-Nazi resistance, actually killed more Jews than Germans during the war.”

The prosecutor’s office said it could not divulge what was said during the questioning, citing the secrecy of the investigation. However, Gross said he was asked to provide information backing up that historical assertion and was also asked if he had intended to insult Poles.

The truth sometimes has a shocking effect on people Jan Tomasz Gross, scholar

“I told him straight that I was not trying to insult the Polish nation. I was trying to raise awareness about the problem of refugees in Europe,” he said. “I am just telling the truth and the truth sometimes has a shocking effect on people who are not aware of what the truth is.”

Jacek Leociak, a historian with the Polish Center for Holocaust Research, said it was difficult to establish exactly how many Jews were killed by Poles during the war but that the number was significant. He said Gross’s comparison could be correct if speaking about Germans killed by Polish underground forces in occupied Poland, and not the Polish army fighting on the western and eastern fronts.

“The claim that Poles killed more Jews than Germans could be really right – and this is shocking news for the traditional thinking about Polish heroism during the war,” Leociak said. He said Gross’s comparison had merit because it “reveals this dimension of the Polish war experience which was always covered, hidden and suppressed”.

President Andrzej Duda is considering stripping Gross of an order of merit he received in 1996. His spokesman, Marek Magierowski, said on Wednesday the president had not yet made a decision.