In reacting to news of the OceanGate submersible implosion, "Titanic" director James Cameron told ABC News that he was "struck by the similarity of the Titanic disaster itself, where the captain was repeatedly warned about ice ahead of his ship and yet he steamed at full speed into an ice field."

Cameron, who has built his own submersible, was critical of OceanGate's use of a carbon-fiber hull, calling it "fundamentally flawed."

OceanGate CEO Stockton Rush, who was among the five passengers killed on the submersible, had previously said he believed a sub made with carbon fiber would have a better strength-to-buoyancy ratio than titanium.

"Titanic" director James Cameron, who has explored the wreckage site himself, called the implosion "quite surreal" and noted that one of the passengers killed, Paul-Henri Nargeolet, was a friend of his.

"For him to have died tragically in this way is almost impossible for me to process," Cameron told ABC News Thursday.

Coast Guard officials said a remote-operated vehicle found the tail cone of the Titan submersible about 1,600 feet from the bow of the Titanic Thursday morning.

Additional debris found was "consistent with the catastrophic loss of the pressure chamber," Rear Adm. John Mauger, the First Coast Guard District commander, said during a press briefing.

The passengers' families were immediately notified.

Five major pieces of debris were found, including the nose cone, officials said.

The debris indicates there was a "catastrophic implosion" of the vessel, Mauger said.

It's too early to tell when the implosion occurred, officials said.

"This is an incredibly unforgiving environment down there on the seafloor," Mauger said.

All five aboard the missing submersible are believed to be lost amid a search for the vessel, OceanGate said.

"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," OceanGate said in a statement.

"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 continued. "Our hearts are with these five souls and every member of their families during this tragic time. We grieve the loss of life and joy they brought to everyone they knew."

Samad Dawood, the brother of Shahzada Dawood, one of the passengers who died in the submersible, said losing his brother and nephew, Suleman Dawood, is heartbreaking.

"This is beyond what you could ever imagine in terms of the kind of hardships and struggle that we've had. I think what we've seen is enormous tragedy and devastation and a lot of emotions. Obviously we were hoping to hear some good news about the rescue and them coming back. It was heartbreaking to find out that that's not the case," Samad Dawood told ABC News.

Samad Dawood said his brother had a strong sense of adventure and curiosity about the world.

"He was always the kind of person who has this incredible love for the world," Samad Dawood said.

"He inspired us curiosity. And he was a guy who just wanted to go out there, experience what the world had to offer, even though he he himself pushed himself to do it. So they made adventures to go to Antarctica, to go to Greenland and to go to Africa and go to the rapids," Samad Dawood said.

This sense of adventure was passed on to Shahzada Dawood's son, 19-year-old Suleman Dawood, who was also in the submersible. Suleman Dawood had traveled to all seven continents, according to Samad Dawood.

"He was so filled with humbleness and gratitude," Samad Dawood said.

"We're deeply grateful for all the support and the love and the effort that the people have made, working tireless nights being optimistic in giving a sense of hope, giving that transparency to us. But obviously we were devastated as a family ... that we couldn't get them back safe," Samad Dawood said.

-ABC News' Octavio Cadenas, Dr. Jennifer Ashton and Victor Ordonez