A confidential assessment issued this week by the Norwegian Shipowners’ Mutual War Risks Insurance Association (DNK) concluded that the attack was likely to have been carried out by a surface vessel operating close by that despatched underwater drones carrying 30-50 kg (65-110 lb) of high-grade explosives to detonate on impact.
https://www.reuters.com/article/us-usa-iran-oil-tankers-exclusive-idUSKCN1SN1P7