President Joe Biden vowed to "find out exactly what happened" after speaking by phone with Polish President Andrzej Duda and NATO Secretary General Jens Stoltenberg about a deadly explosion near Poland’s border with Ukraine.He added that they would “collectively determine our next step as we investigate and proceed.”It was not immediately clear what caused the blast, which followed an intense wave of Russian airstrikes on cities across Ukraine on Tuesday, forcing widespread blackouts and hitting residential buildings in the capital, Kyiv.Biden said he updated leaders at the roundtable meeting — including French President Emmanuel Macron, German Chancellor Olaf Scholz, Canadian Prime Minister Justin Trudeau, Italian Prime Minister Giorgia Meloni, Japanese Prime Minister Kishida Fumio, British Prime Minister Rishi Sunak, European Commission President Ursula von der Leyen and European Council President Charles Michel — about his calls with Duda and Stoltenberg.The Polish Foreign Affairs Ministry issued a statement after the explosion Tuesday saying “a Russian-made missile fell, killing two citizens of the Republic of Poland,” in the southeastern village of Przewodów.Biden, who was briefed about the explosion as he woke up in Bali on Wednesday morning, is expected to leave Indonesia on Wednesday to travel back to Washington."