(CNN) - The matter began with the flow of bubbles rising to the surface in the Baltic Sea. Seven months after the comprehensive Russian invasion of Ukraine in 2022, explosions occurred that destroyed the two Nord Stream gas pipelines - the controversial Russian gas artery to Europe - in the waters between Sweden and Poland.
Investigators from several countries rushed to determine who was responsible, and the finger of blame immediately pointed toward Russia. But later, two Ukrainian citizens were arrested, raising questions about possible state involvement and testing the loyalty of some of Kiev's European allies.
One of them is scheduled to stand trial in Germany, on Wednesday, on charges of involvement in one of the largest - and most mysterious - sabotage operations in modern history. He denies any wrongdoing, and Ukraine has denied any involvement in the incident.
What is the Nord Stream pipeline and what happened?
Nord Stream 1 and Nord Stream 2 are two main routes for transporting Russian gas to Europe via Germany.
Owned by Nord Stream AG, of which the Russian gas company Gazprom is the majority shareholder, these lines had sparked widespread controversy even before the outbreak of war in Ukraine, primarily due to concerns about Europe's dependence on low-cost Russian hydrocarbons.
The “Nord Stream 1” project was announced in 1997 - when relations between the West and Russia in the post-Soviet era were relatively calm - and it entered into operation in 2011. As for the sister project, “Nord Stream 2”, its construction was completed in September 2021, but it never entered service; Germany suspended it days before Moscow invaded Ukraine.
In late September 2022, explosions tore apart two pipelines near the Danish island of Bornholm, spewing clouds of gas into the surrounding waters and triggering a cross-border investigation.
How did the investigation go?
Sweden was the first country to sound the alarm about the leaks.
Some experts at the time indicated that the cause might have been a technical defect, but the leaders of many Western countries were quick to favor the sabotage hypothesis.
Two months after the incident, the Swedish Public Prosecution announced that the incident was the result of a “serious act of sabotage,” noting that “traces of explosives” were found at the site of the incident.
Although neither pipeline was in operation at the time of the attack, they still contained pressurized gas; As a result, experts suggested that “unprecedented” amounts of greenhouse gases would leak into the atmosphere.
Several parties have been tasked with investigating the attack, including security authorities in Germany, Denmark and other Scandinavian countries, while European officials have pledged a firm response.
European Commission President Ursula von der Leyen said that any deliberate action to disrupt Europe's energy infrastructure was "unacceptable" and would result in the "strongest possible response".
Germany, in particular, was keen for the alleged perpetrators to be brought to justice, and in 2024 authorities there issued an international arrest warrant for a Ukrainian man suspected of involvement.
Since then, two Ukrainian men have been arrested for their alleged roles in the attacks on the pipeline.
Both Denmark and Sweden closed their criminal investigations into the explosions in 2024; Sweden attributed its decision to a lack of jurisdiction, while Denmark believed there was insufficient evidence to move forward with the case. Thus, Berlin has become the only country that continues to follow the threads of the case.
Who appears before the court?
Sergey Kuznetsov, a former Ukrainian soldier, is the only defendant in the trial that begins Wednesday in Hamburg, where he is referred to as "Sergey K." Only, in line with German privacy laws.
The German Public Prosecution stated that the accused "belongs to a group of people who planted explosive devices in the 'Nord Stream 1' and 'Nord Stream 2' gas pipelines near the island of Bornholm in September 2022."
The prosecution brought charges against "Sergey K." By complicity in committing a “war crime” of attacking civilian objects, destroying infrastructure, causing an explosion, and disrupting public facilities, according to court documents.
Court documents indicate that he was accused of "acting on behalf of Ukrainian state authorities" in an alleged plot to destroy Nord Stream pipelines.
His defense lawyer previously told CNN that his client denies any wrongdoing.
In 2025, Kuznetsov was arrested by Italian police in Rimini - a city located in northern Italy and famous for its sandy beaches and Roman monuments - and he was later extradited to Germany.
The Public Prosecution in Germany believes that the accused and his accomplices used a sailing yacht to carry out the attack, as they set off from the port of Rostock, located in northern Germany. The prosecution alleges that the boat was chartered from a German company through intermediaries who used false identity documents.
Reuters reported last month that the second Ukrainian man arrested in connection with the bombing, Volodymyr Zhuravlov, had been detained in Croatia in August, where the court agreed to extradite him to Germany. The agency added that his lawyer criticized the ruling, indicating the possibility of appealing the decision.
The German Public Prosecution stated that there is “strong suspicion” that the man was involved in several crimes, including sabotage, causing an explosion, and participating in diving operations to reach pipelines.
Zhuravlov had previously been arrested in Poland, which refused to extradite him to Germany.
How does this relate to the broader war?
The explosions occurred at a time of tension in Europe. These attacks fueled controversy over Europe's dependence on Russian gas, and highlighted Germany, as the driving force behind the gas project, against the backdrop of former Chancellor Angela Merkel's decision to adopt a conciliatory approach towards Moscow, despite its recent aggression against Ukraine and Georgia.
Donald Tusk, the prime minister of Poland (a close ally of Ukraine), said last year that Europe's problem "is not blowing up the Nord Stream 2 pipeline, but building it in the first place."
As early as 2007, Radek Sikorski, then Polish Defense Minister, strongly criticized the Nord Stream 1 pipeline (then under construction), calling it “Mr. Putin’s most crude attempt to split and damage the European Union.”
When Zhuravlov was initially arrested in Poland, a judge ruled against his extradition to Germany, reasoning that the Nord Stream bombings - if they were indeed an act of Ukrainian sabotage - were a justified response to an unjustified invasion.
In light of court officials' expectations that the trial will continue until February of next year, while Germany hopes that no new obstacles will stand in its way, as it seeks to uncover the truth of this mysterious case.
