Poland's military was placed on high alert on Tuesday evening after a missile landed on its territory killing two people.
It came as Russia fired dozens of missiles at targets in Ukraine and Ukrainian forces tried to shoot them down with its own missiles - part of its air defence system.
As the Polish authorities - along with Nato allies - try to establish the facts, the BBC's open source investigations team has been analysing reports and studying images on social media to try to uncover more details about the incident.
Where was the blast?
The explosion was first reported in the Polish media on Tuesday evening, describing a blast close to the Polish village of Przewodow, about 6km (4 miles) from the Ukrainian border.
Initial reports said the missile had landed next to an area of a farm where employees load and weigh grain, destroying a nearby building.
We examined satellite images (which showed only one farm in the village) and cross-referenced them with photographs taken at ground level on Wednesday morning to locate the site - which has been confirmed by the Polish authorities.
We also examined a video which was posted on social media late on Tuesday.
It claims to show the immediate aftermath of an explosion as a missile "fell on Polish territory". A column of smoke can be seen (we're showing a grab of the video on the left of the graphic below).
There are visual clues in the video that we believe appear to match the site in Przewodow.
A road seen in the video (highlighted by us in red) has a similar pattern to one seen in an aerial image of the area - it bends to the left, then to the right.
The road forks in front of a light-coloured building (highlighted by us in orange).
Photographs taken on the ground on Wednesday (seen in the first graphic) show a light-coloured housing block near the farm.
And the vegetation and lines of trees appear similar too.
We've tried to contact the person who posted this video in order to verify it further but haven't yet had a response.
Images from the blast site also began circulating on social media late on Tuesday, showing a crater and a damaged tractor and trailer.
It was difficult to locate these on their own because of the dark and a lack of wider visual clues.
But an image posted on social media by Polish police on Wednesday morning shows officers examining the scene. The same trailer, spilled grain and debris can be seen in both images.
Another picture circulating on social media on Tuesday night - which was widely shared - shows a fragment from a missile that it was claimed was found at the scene.
Based on this image alone, the BBC has so far been unable to verify that it was found at the same location because of the lack of wider visual clues in the photo.
On Wednesday, a BBC correspondent on the ground was unable to get close to the scene, which has been sealed off by the Polish authorities.
We have shown it to defence experts and asked them whether they can work out what type of missile this was from, and who might have fired it.
Mark Cancian, from the think tank CSIS, believes it may be from an S-300 system. This type of missile is typically used for surface-to-air attacks, and has been used by both Russia and Ukraine throughout the war.
"Who fired the missile is unclear," says J Andrés Gannon, a security expert at the US Council on Foreign Relations, who agrees that it may be from an S-300 system.
"We know Russia has been using the S-300 for ground attacks, even though it's an air defence system, but Ukraine also uses them for air defence against cruise missiles."
Dr Justin Bronk, a senior fellow at think tank Rusi, agrees that it may be from an S-300 system, but there isn't enough evidence to identify it yet.
The Polish prime minister said it was highly likely the missile was fired by Ukraine during their defence against Russian attacks.
US President Joe Biden has said it was "unlikely" that the missile was fired from Russia, with US officials' initial findings indicating it was fired by Ukrainian air defences.
Russia's closest point to the blast site in Poland is about 580km (360 miles) away, far further than the range of the S-300 system which experts say is around 90km (56 miles).
How are S300s used?
Both Russia and Ukraine possess these S-300 weapons and accuse each other of causing damage with them in recent attacks.
The Ukrainians use them to shoot down Russian missiles, and the Russians have claimed these have fallen to the ground and caused civilian casualties.
It is very difficult to tell the origin of a missile from its debris, according to weapons experts.
There has been speculation that Russia may have been targeting sites in Ukraine close to this stretch of the Polish border.
About 10km (6 miles) from the village, just across the border into Ukraine, is the Dobrotvirska power plant.
An active power line connecting Poland with Ukraine is also nearby.
Russia has been targeting infrastructure sites such as these and Ukrainian forces have been using anti-missile defences to counter them.
We are continuing to investigate this incident and will update this piece when we get further details.
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