Who blew up the Nord Stream pipelines?

LAST SEPTEMBER, when explosions damaged Nord Stream 1 and 2, two undersea gas pipelines that connected Russia to Germany, evidence of a culprit was scant. Some European governments suspected Russia. President Vladimir Putin had been threatening to cut off energy supplies to Europe in retaliation for sanctions imposed by the EU over Russia’s invasion of Ukraine. Russia, on the other hand, blamed America and Britain. Six months later American intelligence officials, as well as German and British news organisations, have suggested the possible involvement of an unidentified pro-Ukrainian group unaffiliated to the government. So whodunnit, and why?