HAMBURG, Germany (AP) — A former Ukrainian army officer went on trial in Germany on Wednesday over the undersea explosions four years ago that damaged the Nord Stream gas pipelines linking Russia and Germany in what is considered one of Europe’s biggest acts of sabotage since World War II.

In a trial expected to last months, the suspect, Serhii Kuznietsov, faces charges of causing an explosion, damaging property, disrupting public service and being an “accomplice to war crimes” by attacking civilian infrastructure, according to prosecutors.

Kuznietsov, dressed in a gray suit and dark tie, kept a calm expression as he sat between two lawyers in black robes as the proceedings got underway at the Hanseatic Higher Regional Court in Hamburg.

The trial of the suspect, who is believed to be the alleged ringleader of the attack, is being held under strict security measures.

While international observers initially blamed Moscow for the blasts, investigators later found suggestions that Ukrainian state security agencies may have been involved — even though Ukraine counts Germany as one of its closest allies.

More than 100 media representatives from around the world, including Britain, China, France, Ukraine and Russia, have been accredited to cover the case, according to the court.

Pro-Ukraine protesters gathered outside the courthouse as attendees filed into the building, holding up placards and signs that carried messages including “Russia is a Terrorist” written in red.

Ivan Kuritsyn of Citizens’ Initiative Vilni De Ua, a nonprofit formed by Ukrainians displaced by the war, said the demonstrators were on hand “to show our solidarity with Serhii Kuznietsov and to support him."

“I want to emphasize here that we in no way question the court’s decision, because only the court can decide whether Serhii is guilty or not, but we must support him regardless,” Kuritsyn added.

Explosions that damaged the pipelines at center of case

According to the charges, the defendant was allegedly involved in causing the Sept. 26, 2022, explosions that damaged pipelines built to carry Russian natural gas to Germany under the Baltic Sea.

At the time, European countries were moving to wean themselves off Russian energy sources, following the Kremlin’s full-scale invasion of Ukraine in February 2022.

Prosecutors say the aim of the plot was to permanently disrupt gas deliveries and prevent Russia from using the revenues to finance its military operations.

The defendant and others allegedly used a yacht that departed from the German port of Rostock. It had been rented from a German company using forged IDs and with the help of intermediaries.

The explosions ruptured the Nord Stream 1 pipeline, which was Russia’s main natural gas supply route to Germany until Moscow cut off supplies in August 2022. They also damaged Nord Stream 2, which never entered service because Germany suspended its certification process shortly before Russia’s invasion of Ukraine.

The suspect allegedly organized and carried out the detonation of at least four bombs between 14 and 27 kilograms (around 31 to 62 pounds) at a depth of 70 to 80 meters (230 feet to 263 feet) near the Danish island of Bornholm.

Kuznietsov was detained last year in an Italian village, where officers raided a bungalow where he was staying with his family. Police said he surrendered without resistance. He was publicly named in extradition proceedings in Italy that led to his transfer to Germany in November.

Ukraine has denied any role in the pipeline blasts

A second suspect was arrested on a European warrant in the northern Croatian town of Pula in August. He has applied for asylum in Croatia to fight his extradition to Germany.

The pipelines were long a target of criticism by the U.S. and some of its allies, which warned that they increased dependence on Russian gas.

Russia has claimed that U.S. and British intelligence agencies were involved in staging the explosions and has also accused NATO of involvement in the attack, while several Western countries initially blamed Russia.

Ukrainian President Volodymyr Zelenskyy has denied allegations that his country was involved in the pipeline blasts.

The German government has so far refrained from comment on the trial, pointing out that no verdict has yet been reached and the case is in the hands of the independent judiciary.

So far, 16 court dates have been scheduled through Dec. 17. Additional dates are also planned for January and February 2027. It is not yet known when the verdict will be announced.