The United States and its Western allies are scrambling to investigate after a missile landed in Poland, killing two people.
Ukraine initially claimed it was a Russian rocket that exploded in the Polish village of Przewodow, which is around 6 kilometres from the border of Ukraine.
However, Russia's defence minister denied the missile was theirs, describing the reports as a deliberate provocation.
After first suggesting Russia was to blame, US officials and President Joe Biden have since said the rocket was probably not fired from within Russian territory.
Meanwhile, Poland's President Andrzej Duda said the country still had no concrete evidence showing exactly who fired the rocket.
Regardless, with Poland being a member of the North Atlantic Treaty Organization (NATO), countries are on high alert.
The incident, whether intentional or accidental, has raised the risk of widening the war in Ukraine by drawing other countries directly into the conflict.
This is what we know so far.
Can missiles just go rogue?
Sonia Mycak, a research fellow at the ANU's Centre for European Studies, said although this was the first time a missile had crossed into Polish territory, they have come very close before.
"We saw in mid-June an escalation whereby missiles landed within 15 kilometres of the Polish border," Dr Mycak told the ABC.
"So I don't actually see this as something new."
Marcus Hellyer, a senior analyst at the Australia Strategic Policy Institute, said whether a missile could "go rogue" depended on the technology and the era of the missile.
There is still speculation about what kind of missile was involved — both the KH-101 and the S-300 have been named.
Dr Hellyer said the S-300 was a Soviet or Russian-pedigree air-defence missile that both Ukrainians and Russians used.
It might have been a Ukrainian missile that had gone astray.
Ukraine has been using S-300s to shoot down cruise missiles and other ordinance Russians have been using quite prolifically recently, Dr Hellyer says.
But, there are other theories.
"There's pretty good evidence that because Russia has been running low on dedicated land-attack missiles they have actually been repurposing their own S-300s from an air-defence role into a land-attack role," Dr Hellyer said.
"At the moment, it's probably too premature to speculate along the lines of whether this is a deliberate Russian provocation, let alone whether this is a Russian missile that's gone off course."
Experts should be able to tell who the missile belongs to based on the serial number and where it was manufactured.
But even then, it may not tell the whole story.
Hardware may be captured by one side and put into service by the other, Dr Hellyer said.
"But at some level, it's irrelevant," he said.
"A country has every right to defend itself. So even if it's a rogue Ukrainian air-defence missile, they have every right to be defending themselves.
"So ultimately, it still comes down on Putin."
So far, Poland has only said the missile that fell into the eastern village is Russian-made.
"We just have to wait for the pictures," Matthew Sussex, senior fellow at the Centre for Defence Studies, said.
While the S-300 is used by both Kyiv and Moscow, the KH-101 is a long-range cruise missile that Russia has specifically been using.
"Basically, it's what the Russians have been shooting off at the Ukrainians," Professor Sussex said.
"It could be just a cruise missile from a ship or a submarine, or from Russian territory in Crimea, or it could also be launched from an aircraft."
US President Joe Biden told reporters on Wednesday it was "unlikely in the lines of the trajectory that it was fired from Russia. but we’ll see".
Three US officials said preliminary assessments suggested Ukrainian forces fired the missile at an incoming Russian rocket amid the crushing salvo against Ukraine's electrical infrastructure on Tuesday.
The officials spoke on condition of anonymity because they were not authorised to discuss the matter publicly.
What does the location tell us?
Russia was pounding cities across Ukraine with missiles on Tuesday, in attacks that Kyiv said were the heaviest wave of missile strikes in nearly nine months of war.
Some hit Lviv, which is less than 80km from the border with Poland.
Professor Sussex said guidance systems for Russian missiles had been notoriously bad since the war started.
"They call them precision guidance systems but only about 70 per cent would be accurate," he said.
He added that it was possible that one went off course after being aimed at power infrastructure in Lviv.
Dr Hellyer said if a missile was to go off course, it was either unstable, struck by another missile, or just malfunctioning.
In any case, they likely ended up close to where they were launched in the first place, he said.
"On the whole, they're not terribly airworthy so they're probably going to crash pretty close to where they were launched," Dr Hellyer said.
"It's the exception where a missile sort of just heads off into the wild blue yonder and lands a long way away."
Missiles a sign of 'escalation'
Ukraine's President Volodymyr Zelenskyy said the missile hitting Poland was a "significant escalation" in his country's conflict with Russia.
While Poland's president said there was no "conclusive evidence" yet as to who launched the missile, "it was most likely a Russian-made missile, but this is all still under investigation at the moment".
Dr Mycak said the word coming out of Ukraine was that this was no coincidence.
"Poland has been perhaps the strongest European partner to Ukraine, in terms of fighting Russian aggression," she said.
"And certainly within Ukraine, the feeling is that this is not a mistake."
She believes Russian President Vladimir Putin may be testing NATO's article five to see how quick and severe a reaction may be.
Dr Hellyer also said it was surprising there had not been more of these incidents during the nearly nine-month invasion of Ukraine.
"I think it is quite remarkable that we haven't seen more in terms of a horizontal escalation where the war spreads into other countries, whether by accident or by intent," he said.
NATO's article five is an agreement that an armed attack against one or more NATO countries is considered an attack against them all.
"There seems to be a sort of an assumption that NATO countries will immediately step into a military response," Dr Mycak said.
"I don't think that that is the case. How each individual NATO country responds to the attack is actually still a matter for the individual country, and what that country deems necessary."
Regardless of whether the incident was deliberate or a mistake, it shows how dangerous and volatile the conflict is to neighbouring countries.
"This could happen at any time again," Dr Mycak said.
"This event shows that other countries are going to be more and more affected by what's going on in Ukraine."
