Out on the western Baltic, a coastguard officer radios a nearby, sanctioned oil tanker.
Swedish Coastguard calling… Do you consent to answer a few questions for us? Over.
Through heavy static, barely audible answers crackle over from a crew member, who gradually lists the ship's insurance details, flag state and last port of call – Suez, Egypt.
I think this ship will go up to Russia and get oil, says Swedish investigator, Jonatan Tholin.
This is the front line of Europe's uneasy standoff with Russia's so-called shadow fleet; a term that commonly refers to hundreds of tankers used to bypass a price cap on Russian oil exports.
After the Kremlin launched its full-scale invasion of Ukraine, many Western countries imposed sanctions on Russian energy, which Moscow is accused of dodging by shipping oil on aged tankers often with obscure ownership or insurance.
Some shadow ships are even suspected of undersea sabotage, illicit drone launches or spoofing their location data.
Out on the waves, where freedom of navigation is a golden rule, the ability and appetite of coastal countries to intervene is limited, even though the risk they face is escalating.
As the BBC has learned, a growing network of shadow ships are sailing without a valid national flag, which can render vessels stateless and without proper insurance.
That is a troubling trend, given many are practically floating rust buckets, says senior maritime intelligence analyst at Windward AI, Michelle Wiese Bockmann. If there is an accident, like a billion-dollar oil spill, good luck with trying to find somebody responsible to pick up any cost.
Driven by record sanctions and tighter enforcement, the number of falsely flagged ships globally has more than doubled this year to over 450, most of them tankers, according to the International Maritime Organization (IMO) database.
The BBC has been tracking one ship that appears to have sailed without a valid flag.
The head of Estonia's navy, Commodore Ivo Värk, highlights the increase in such vessels passing through essential shipping routes, noting a marked rise in suspicious maritime activity since the beginning of the conflict.
Despite the emergence of these shadow vessels, European enforcement efforts remain constrained by maritime laws that protect innocent passage, complicating direct action against non-compliant vessels.
Even amidst ongoing geopolitical tensions, the risk of escalation remains high, leading many countries to cautiously monitor rather than aggressively intervene against suspected shadow fleets.
As enforcement of sanctions continues to evolve, the international community is urged to develop strategies that address the operational challenges posed by these elusive vessels, thereby bolstering maritime security in the Baltic region.
