Two Polish citizens are dead after a missile reportedly landed 15 miles from the Ukrainian border, leading to speculation as to which nation is responsible.

Although Polish President Andrzej Duda claimed that the armaments were “Russian-made,” he also suggested, “What happened was an isolated incident … There is no indication that more will take place.”

Russia’s ministry of defense has denied any involvement with the incident, stating, “No strikes on targets near the Ukrainian-Polish state border were made by Russian means of destruction … statements of Polish media and officials [are] a deliberate provocation aimed at escalating the situation.”

Nevertheless, Ukrainian President Volodymyr Zelenskyy alleged, “What we warned about a long time ago happened today. We’ve been telling this. Terror is not limited to our national borders. … today, Russian missiles hit Poland.”

Because Poland is a member of NATO, a Russian attack within its border could trigger the invocation of Article 5, which provides for the collective defense of any member nation. Such an action would functionally ignite Russian-American hostilities.

Other nations in the region have increased defense preparations in response to the explosions within Polish borders, including Hungry, Slovakia, Latvia, and the Czech Republic.

American President Joe Biden, however, has been skeptical of the incident, responding to a question regarding the alleged Russian origin of the missiles saying, “There is preliminary information that contests that. I don’t want to say that until we completely investigate it.”

“It is unlikely in the lines of the trajectory that it was fired from Russia but we’ll see,” Biden added.

Some popular media personalities who took to Twitter to cover the conflict have suggested that pictures of the missile debris indicate that the rockets might have been models commonly used by the Ukrainian military in an air defense role.

A so-called Ukrainian whistleblower reportedly claimed that the debris was part of Ukrainian anti-air missiles that had been attempting to intercept a Russian cruise missile.

The Guardian reported that some analysts said missile debris photographed at the scene appeared to be from a Ukrainian S-300 air defense system.

The investigations into the incident are ongoing.