Specifically an IRGC surface vessel is believed to have launched the underwater drones packed with between 30 and 50kg of explosives which detonated on impact, according to a new report issued this week by the Norwegian Shipowners' Mutual War Risks Insurance Association, known as DNK. Among the vessels hit were a Norwegian-flagged vessel as well as a UAE ship. 

A new report by Reuters summarized the Norwegian insurance investigators' preliminary findings as based on analyzing shrapnel from the attacks which was "similar" to shrapnel recovered from surface drones used off Yemen by Iran-backed Houthi militia.

However, the insurance assessment seen by Reuters is "confidential" with an investigation still ongoing, and thus must be treated with skepticism.

Further, the evidence appears largely circumstantial at this point, with Iran's guilt appearing to hinge on the assumption that shrapnel from Houthi operations and remnant material found at the port of Fujairah are from the same source.