Germany arrested a Ukrainian man in Italy on Thursday in connection with the mysterious Nord Stream gas pipeline explosions, a massive act of sabotage that remains under investigation.

The man, identified as Serhii K, is part of a Ukrainian group that planted explosives in Swedish and Danish waters under the Nord Stream 1 and Nord Stream 2 pipelines, federal prosecutors said. He is “believed to have been one of the coordinators of the operation” who set off with accomplices from Rostock on Germany’s northeastern coast in a sailing yacht to carry out the attack, they added.