BOSTON — A renowned Titanic expert, a world record-holding adventurer, two members of one of Pakistan’s wealthiest families and the CEO of the company leading an expedition to the world’s most famous shipwreck were killed aboard the Titan submersible when it imploded in the Atlantic Ocean sometime this week.

       The U.S. Coast Guard on Thursday said there were no survivors after the catastrophic implosion deep in the North Atlantic.

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       The search for the submersible and its occupants — as well as any clues to explain what happened underwater — were ongoing Thursday after a deep-sea robot found debris near the Titanic shipwreck.