U.S. President Joe Biden and key European leaders are urging caution after a rocket struck a Polish village just over the border from Ukraine, as doubts grow over whether Russia was to blame and, if so, whether it had been an accident.

The attack, which killed two people at a village about 6 kilometers from the frontier with Ukraine late Tuesday, comes as tensions are already high with Russia over its February invasion of Ukraine. Moscow fired a missile barrage at Ukrainian infrastructure earlier in the day, causing energy outages across the country.

Some leaders initially suggested the rocket that landed in Poland had been launched by Russia, but by Wednesday morning the picture was less clear, with some even suggesting it could have been Ukraine’s missile defense sending a Russian projectile off course.