Poland's president said there was no clear evidence of who fired the missile that killed two people in a Polish village on Tuesday, adding that it was "most probably Russian-made".
"We do not for the moment have unequivocal evidence of who fired the missile. An investigation is ongoing. It was most probably Russian-made," President Andrzej Duda told reporters on Wednesday.
Duda said it was an "isolated" incident.
"Nothing indicates to us that there will be more," he said.
The foreign ministry earlier said the missile was Russian-made and summoned Russia's ambassador to Warsaw to give "immediate detailed explanations".
The ministry said the meeting with the Russian ambassador ended "before midnight" but gave no further detail.
Ukraine has blamed Russia, which launched a wave of missile strikes across Ukraine that left millions of households without power.
Duda said he had spoken to US President Joe Biden who had promised "support in the form of American experts to help us investigate at the site of this tragic incident".
He also said it was "highly likely" that Poland's ambassador to NATO will request urgent consultations under Article 4 of the NATO Treaty at a meeting with other alliance ambassadors in Brussels on Wednesday.
Article 4 of the NATO Treaty states that consultations can be called when any NATO member feels their "territorial integrity, political independence or security" are at risk.
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