"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."

A Royal Canadian Navy ship HMCS Glace Bay has been on scene since about 8 a.m. ET Thursday morning. The ship provides a medical team specializing in dive medicine and a six-person mobile hyperbaric recompression chamber, according to officials.

Canadian Coast Guard ships John Cabot, Ann Harvey and Terry Fox are on scene and ready to provide rescue equipment and personnel should assistance be required. A Royal Canadian Air Force CP-140 Aurora aircraft is also maintaining continuous on-scene support with additional aircrews and assets, according to officials.

-ABC News' Luke Barr

Explorer Robert Ballard, who was on the 1985 expedition that discovered the Titanic wreckage and has made many dives since, reacted to news of the implosion with ABC News on Thursday.

"It's very tragic what happened, I know the people that passed away," he said, adding that he's known OceanGate CEO Stockton Rush, who was piloting the vessel, for 20 years. "I know this was his dream, so my heart goes out to him and his family and the families of all the people who lost loved ones."

Ballard surmised the crew likely experienced difficulties and began to drop weights to ascend to the surface but "never made it."

"If you're on your way up and you're buoyant and you don't make it, it's a catastrophic implosion," he said.

"I don't think people can appreciate the amazing energy involved in the destructive process of an implosion," Ballard continued. "It just takes and literally shreds everything. So it's extremely powerful."