Putin says it was Ukrainian special services. Well, duh. Maybe they were helped by Westerners too. In a war it is normal to try to destroy each others infrastructure. And since Westerners are propping up Ukraine so can hold out, it would be normal to give intel support too (as they do anyway).
https://www.aljazeera.com/news/2022/10/10/putin-accuses-ukraine-of-crimea-bridge-blast-terrorism