US officials have not ruled out the possibility that the tankers involved in the Gulf of Oman incident Thursday were attacked by a projectile or hit an underwater mine.
"The US at this hour has not ruled out [that] the ships may have hit a mine in the water, or were attacked by a projectile. They are trying to determine the cause," a US defense official told CNN.
A US P8 Poseidon maritime patrol aircraft is conducting surveillance in the area, according to the same source.
The guided missile destroyer USS Bainbridge responded to the incident after the US Fifth Fleet -- responsible for US naval forces in the area -- received two separate distress calls on Thursday morning.
The US defense official said the destroyer took on board 21 mariners from the Kokuka Courageous tanker, who were originally rescued by a tug boat.