It was carrying British billionaire Harding and dual Pakistani-British citizens Dawood, a tycoon, and his son Suleman.
OceanGate Expeditions charges $250,000 for a seat on the sub.
Also on board was OceanGate's CEO Rush and French submarine operator Nargeolet, nicknamed "Mr Titanic" for his frequent dives at the site.
The operator of a submersible missing near the wreck of the Titanic said Thursday it believed all five people on board were dead, due to what the US Coast Guard described as a "catastrophic" implosion of the vessel in the ocean depths.
The solemn announcement from Oceangate came hours after rescuers said a "debris field" had been discovered following a dayslong search -- adding to fears that, even if the vessel were still intact, its oxygen may have run out.
