In the absence of any official announcement, and reading between the lines from various different sources, I have picked up two hints as to what happened to the ships anchored outside Fujairah.
One article says they were limpet mines, and another said that the 'shrapnel' found was of the type used by Iran's Revolutionary Guards.
A Japanese tanker was attacked in the Persian Gulf from behind in similar mysterious circumstances some years ago. At first sight no explosive residue. The Japanese ship looked as though it had been punched by a giant fist. The latest Norwegian ship looks like someone took a can opener to the stern at the water line. If these were meant to be a warning, with no loss of oil, then perhaps the explosive charge was lessened in some way. (?) Since 'they' hit ships 'to the east' of Fallujah in the Gulf of Oman, that would suggest to me that their escape route lay on that side.
The Houthis in Yemen probably do not yet have quite that reach. And if the US was threatening to cut off Iran's oil exports and passage through the Strait of Hormuz, then it would be logical for Iran to give the nod to any entity able to hit the end of the UAE's overland Hormuz-bypassing pipeline operations in Fujairah, if only as a tit-for-tat warning.