A Ukrainian citizen is wanted over the 2022 attack on the Nord Stream gas pipelines, it was revealed on Wednesday.
Prosecutors in Germany have an arrest warrant for the man, believed to be a diving instructor who lived in Poland until leaving for Ukraine last month.
Authorities in Poland confirmed the existence of the arrest warrant after it was revealed by several German media outlets.
Their investigation does not allege that Ukrainian military or intelligence services were behind the attack on the Russian-owned pipelines.
But the arrest warrant marks the biggest breakthrough yet in a case that has puzzled investigators for almost two years.
Several theories had been proposed for the explosions on the Russia-to-Germany pipelines that caused gas leaks in Danish and Swedish waters.
Many suspected a Kremlin ploy to cause a gas shortage in Europe or send a warning to the West, in keeping with a history of undercover Russian operations.
Russia denied involvement, and none of Ukraine's allies have come forward with concrete evidence against Moscow.
Suspicion also fell on Ukraine and the US, who might have benefitted from destroying one of Russia's key levers of influence over Europe.
Both countries denied any role in what would have been a high-risk act of sabotage in the waters of Ukrainian allies.
The new report by German broadcaster ARD and newspapers Die Zeit and Sueddeutsche Zeitung says three Ukrainians are suspects in the case.
The man under an arrest warrant is believed to have lived in Poland until recently but currently remains at large.
He denied involvement when contacted by media outlets.
The suspect's name was reported as Volodymyr Z. He is not the Ukrainian President, Volodymyr Zelenskyy.
Married suspects
Two more Ukrainians, a married couple who run a diving school, are said to be suspects in the German investigation.
It is believed a sailing yacht called Andromeda may have been used to reach the Nord Stream 1 and 2 pipelines under the Baltic Sea.
Detectives traced a white van that was caught speeding near the Baltic coast and may have brought diving supplies to the yacht, it is reported.
The van driver is said to have identified Volodymyr Z as a passenger when spoken to by German investigators.
It was not clear why Polish police have not arrested Volodymyr Z, who is now believed to have gone underground.
Germany is said to suspect him of “deliberately causing an explosion” and “sabotage hostile to the constitution”. Sweden and Denmark have closed their investigations.
Neither pipeline was functioning at the time of the explosions, after Russian exporter Gazprom cut off gas through Nord Stream 1.
The controversial Nord Stream 2 never went into operation, having been suspended by German Chancellor Olaf Scholz shortly before Russia invaded Ukraine.
Ukraine and Poland had heavily lobbied against the opening of Nord Stream 2, fearing it would be used for Russian blackmail.
The previous German government had dismissed security fears and described the project as purely economic.
The Danish military reported last year that it had spotted a Russian submarine rescue ship in the Baltic a few days before the blasts. The Kremlin branded claims that its own forces blew up a major revenue stream as “stupid”.
