Tuesday's explosion that killed two people in the village of Przewodów has heightened fears that Russia's war in Ukraine could spread and drag in NATO.
Key questions remain around the circumstances of the missile which struck Poland near its Ukrainian border on Tuesday, killing two people.
None is larger than who fired it.
The incident took place on a day when Russia launched a blistering series of airstrikes across Ukraine, but Moscow denied any involvement in the Polish blast.
Fears surged that a deliberate, hostile attack on NATO member Poland could trigger a collective military response by the alliance.
Here is an outline of what we know, what we do not know, and how NATO could react to the explosion in Poland:
Where did the missile come from?
This remains unclear, but it is becoming increasingly unlikely that Russia fired the missile.
NATO chief Jens Stoltenberg said on Wednesday that the Poland blast was likely caused by Ukrainian air defence trying to defend itself from a Russian missile attack.
Poland president Andrzej Duda appeared to share this view.
"There is a high probability that this is a missile that was simply used by Ukrainian missile defence," he said, adding it was "probably an unfortunate accident".
Poland prime minister Mateusz Morawiecki stated on Wednesday that there are "many indications" that the blast was caused by a missile launched by Kyiv forces.
"Yesterday, countering Russia's massive missile attack on Ukrainian territory, Ukrainian forces fired missiles to shoot down Russian (ones). There are many indications that one fell on Polish territory, without any intention of either party," Morawiecki said at a press conference, according to his office.
Following a meeting with the alliance envoys, Stoltenberg said at a press conference that "our preliminary analysis suggests that the incident was likely caused by a Ukrainian defence missile fired to defend (its) territory against Russian cruise missile attacks."
"But let me be clear, this is not Ukraine’s fault," Stoltenberg added. "Russia bears ultimate responsibility as it continues its illegal assault against Ukraine."
President Joe Biden said earlier Wednesday it was “unlikely” that a missile that killed two in NATO-ally Poland was fired from Russia.
“There is preliminary information that contests that," Biden told reporters at the G20 summit in Indonesia when asked if the missile had been fired from Russia. “It is unlikely in the lines of the trajectory that it was fired from Russia, but we’ll see.”
Speaking anonymously, three US officials said preliminary assessments suggested the missile was fired by Ukrainian forces at an incoming Russian one amid the crushing salvo against Ukraine’s electrical infrastructure.
That assessment and Biden’s comments at the G20 summit contradict information earlier Tuesday from a senior US intelligence official who told the AP that stray Russian missiles crossed into Poland.
In an investigation like this, information like radar data, ammunition serial numbers, and other identifiers retrieved from the impact site could prove decisive, but analysing it could take time.
How did Poland manage the crisis?
The Polish government acted cautiously and asked for restraint until more information came to light.
Warsaw said by Tuesday evening it was investigating the blast and raising its level of military preparedness. A statement from the Polish Foreign Ministry identified the weapon as being made in Russia.
Duda was more careful, saying that it was “most probably” Russian-made but that its origins were still being verified.
What kind of a missile was it?
The information available is inconclusive, but most statements by officials and analysis of images from the scene point to the missile being identified as part of the Soviet-designed S-300 air-defence missile system.
Introduced in 1979 during the Cold War, S-300's intended use was to take down incoming cruise missiles targeting infrastructure or military compounds. Some of the later variants were made to intercept ballistic missiles as well.
It is fully automated, although there is a mechanism that allows it to be operated manually. The range depends on the missiles used with the system, but the standard 48N6 missile can strike targets at any range between five and 150 kilometres.
The system and several of its more modern variants are used by both Russia and Ukraine, together with a number of former Warsaw Pact and other European countries, such as Belarus, Bulgaria and Greece.
The more contemporary versions of S-300 are said to be comparable to the US-made Patriot system.
In its full-scale invasion of Ukraine, Russia has used S-300 systems to attack ground targets. At the same time, Ukraine actively uses the same system for air defence purposes.
Was it an accident, or was it deliberate?
Stoltenberg said on Wednesday that there were no signs the blast in Poland caused by a missile was an intentional strike by Russia.
"An investigation is ongoing, and we need to await its outcome, but we have no indication that this was an assault or a deliberate attack," Stoltenberg said, adding that there is no proof that "Russia is preparing offensive military actions against NATO."
Poland’s initial statement did not address whether the strike could have been a targeting error or a technical fault or if an incoming missile could have been knocked off course by Ukrainian defences.
If Russia had deliberately targeted Poland, it would risk drawing the 30-nation alliance into the conflict at a time when it is already struggling to fend off Ukrainian forces.
How did Moscow react?
The Russian Defence Ministry denied being behind “any strikes on targets near the Ukrainian-Polish border” and said in a statement that photos of purported damage “have nothing to do” with Russian weapons.
According to Moscow, the photos of the wreckage in Przewodow "are unequivocally identified by Russian defence industry specialists as elements of an anti-aircraft guided missile of the S-300 air defence system of the Ukrainian air force," the Russian defence ministry said in a statement on Wednesday.
This claim could not be independently verified.
Additionally, blasts were reported in the western Ukrainian city of Lviv earlier on Tuesday, implying that Russia has targeted parts of Ukraine bordering Poland.
Lviv is located some 65 kilometres from the Polish border, while the village of Przewodow is just 6 kilometres from Ukraine, albeit further north of Lviv.
Although some analysts have pointed out that Przewodow is in close proximity to Ukraine's Dobrotvirska power plant, there is no proof at this time that the explosion in Poland could have been directly related to Russian strikes on Ukrainian infrastructure.
What about the Ukrainian reaction?
After the blast, Ukraine's top officials pointed the finger squarely at Russia.
In a video message Tuesday evening, Ukrainian President Volodymyr Zelenskyy slammed the Kremlin for what he said were "Russian missiles (hitting) Poland, the territory of our friendly country."
"Poland, the Baltic states... It’s only a matter of time before Russian terror goes further," he remarked, adding that the alleged Russian strike on "NATO territory is ... a Russian missile attack on collective security."
Reacting to the news, Foreign Minister Dmytro Kuleba said the "collective response to Russian actions must be tough and principled".
Kuleba went on to propose a NATO summit with Ukraine's participation which will force Russia to change its course on escalation," adding that Kyiv should be provided with "modern aircraft such as F-15 and F-16, as well as air defence systems."
"Today, protecting Ukraine’s skies means protecting NATO,” he said in a series of tweets.
Ukraine is not a NATO member. It applied to join the alliance earlier in the war -- but the decision to consider Ukraine for membership rests squarely in the hands of all 30 alliance members.
However, the Ukrainian stance might change if it is proven that the missile originated from their own air defence systems.
Kuleba dismissed the possibility on Tuesday evening as "Russian propaganda". Stoltenberg refused to comment on Kuleba's words on Wednesday, repeating that "it was not Ukraine's fault".
Following the NATO chief's press conference, a senior Ukrainian defence official said Kyiv wants access to the site of an explosion in eastern Poland, claiming it had evidence of a "Russian trace" in the incident.
Oleksiy Danilov, Secretary of the National Security and Defence Council of Ukraine, said Ukraine wanted a joint study of Tuesday's incident with its partners and to see the information that provided the basis for its allies' conclusions.
Kyiv is "completely open to a comprehensive study of the situation," he wrote on the council's official Facebook page. Danilov provided no details of what evidence he was citing.
The Polish president responded that both Poland and the US would have to agree to Ukraine taking part in the investigation first.
What did Russian strikes target on Tuesday?
With its battlefield losses mounting, Russia has increasingly resorted to targeting Ukraine’s power grid, seemingly hoping to turn the approach of winter into a weapon by leaving people in the cold and dark.
Russia pounded Ukraine’s energy facilities with its biggest barrage of missiles yet, striking targets across the country and causing widespread blackouts.
The barrage also affected neighbouring Moldova. It reported massive power outages after the strikes knocked out a key power line that supplies the small nation, an official said.
The missile strikes plunged much of Ukraine into darkness and drew defiance from Zelenskyy, who shook his fist and declared: “We will survive everything.”
In his nightly address, the Ukrainian leader said the strike in Poland offered proof that “terror is not limited by our state borders.”
Russia fired at least 85 missiles, most of them aimed at the country's power facilities, and blacked out many cities, he said.
The Ukrainian energy minister said the attack was “the most massive” bombardment of power facilities in the nearly 9-month-old invasion, striking both power generation and transmission systems.
What is happening diplomatically?
President Biden held an emergency meeting with leaders of the G7 -- which includes Canada, France, Germany, Italy, Japan, the United Kingdom and the European Union -- along with the president of the European Council and the prime ministers of NATO allies Spain and the Netherlands.
The UN Security Council also planned to meet on Wednesday for a previously scheduled briefing on the situation in Ukraine.
Leaders of the G20 nations deplored Russia's aggression in Ukraine "in the strongest terms" on Wednesday and demanded its unconditional withdrawal in a declaration adopted at the end of a two-day summit.
Polish Foreign Minister Zbigniew Rau summoned the Russian ambassador and “demanded immediate detailed explanations,” the government said.
Some have called for caution, including German Chancellor Olaf Scholz, who emphasised that "any premature determination is out of the question".
"One thing is clear: none of this would have happened without Russia's war of aggression against Ukraine," he wrote on Twitter.
Other world leaders have sided with Russia on this issue.
Turkish President Recep Tayyip Erdogan said on Wednesday that he respects Russia's statement that Russian missiles had not hit Polish territory, adding that he believes Moscow had "nothing to do with it".
"Russia saying this has nothing to do with them and Biden saying these missiles are not Russian-made show that this has nothing to do with Russia," Erdogan said at a news conference during the G20 summit.
Erdogan said the situation needs to be investigated and that he will speak with Russian President Vladimir Putin when he returns to Turkey.
What could NATO do?
In the aftermath of its government's emergency meeting on Tuesday, Poland mulled triggering Article 4 of the NATO founding treaty, which would result in a meeting of all member states after an ally's territorial integrity or security has been breached.
This was pre-empted by Stoltenberg's meeting with NATO ambassadors.
If it is determined that Moscow intentionally targeted Poland, Warsaw has the option of invoking NATO's principle of collective defence known as Article 5, in which an attack on one of the Western alliance's members is deemed an attack on all, starting deliberations on a potential military response.
However, this seems highly unlikely in the aftermath of Stoltenberg's press conference in Brussels.
As more information about the incident came to the fore on Wednesday, NATO and its members appeared to be looking to de-escalate the situation instead.
Poland has also taken individual steps to protect its territory, placing select units on alert, mostly in its air force.