U.S. Navy ships and aircraft combed areas of the Gulf of Aden for two missing U.S. Navy SEALs on Monday as details emerged about their mission to board and take over a vessel carrying components for medium-range Iranian ballistic missiles headed for Somalia, a U.S. defense official said Monday.

The official said crew on the dhow, which did not have a country flag, were planning to transfer the missile parts, including warheads and engines, to another boat off the coast of Somalia. The Navy recognized the boat as one with a history of transporting illegal weapons from Iran to Somalia, said the official, who spoke on the condition of anonymity to discuss details not made public.