OceanGate Expeditions said on Thursday that they believe five crew members on board the Titan submersive that went missing on Sunday didn't survive.

“We now believe that our CEO Stockton Rush, Shahzada Dawood and his son Suleman Dawood, Hamish Harding, and Paul-Henri Nargeolet, have sadly been lost,” a spokesperson told BBC News.

“These men were true explorers who shared a distinct spirit of adventure, and a deep passion for exploring and protecting the world’s oceans," the statement added.

Debris discovered on the ocean floor suggests the missing submersible near the wreck of the Titanic suffered a "catastrophic loss" of pressure, the U.S. Coast Guard said Thursday.

"Upon this determination, we immediately notified the families," Rear Admiral John Mauger told reporters in Boston.

"On behalf of the United States Coast Guard and the entire unified command, I offer my deepest condolences to the families."

Earlier on Thursday, a "debris field" was discovered by an underwater robot. According to Carl Hartsville of the Woods Hole Oceanographic, it was found over 650 ft from the bow of the Titanic wreck.  

The missing submersive was estimated to run out of oxygen on Thursday morning, based on the sub's capacity to hold up to 96 hours of air. Earlier on Wednesday, "banging" sounds were detected underwater during the search but the U.S. Coast Guard later admitted that it was unable to identify their source.

The 21-foot submersive was carrying British billionaire Hamish Harding, as well as Pakistani tycoon Shahzada Dawood and his son Suleman, who also have British citizenship. It lost communication with its mothership less than two hours into its descent Sunday to see the remains of the Titanic, which sits more than two miles below the surface of the ocean.