"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

A submersible on a tour of the Titanic wreckage was reported overdue by its operator OceanGate Expeditions on Sunday, prompting the United States Coast Guard to launch a search and rescue effort for the 22-foot, 23,000-pound vessel.

Designed with life support to sustain five crew members for 96 hours, the submersible would need to be rescued in three days to save its five passengers, according to the Coast Guard.

Stockton Rush founded Washington-based OceanGate in 2009 to make deep-sea exploration more accessible to scientists and tourists. Fourteen years, more than 200 dives and three submersible designs later, the company now finds itself in a desperate search to recover the submersible carrying five people aboard that’s gone missing off the coast of Newfoundland, Canada.

OceanGate confirmed Monday it had lost contact with a submersible, saying in a statement: "We are exploring and mobilizing all options to bring the crew back safely. Our entire focus is on the crewmembers in the submersible and their families. We are working toward the safe return of the crewmembers."

Here's what we know about the submersible.

-ABC News' Peter Charalambous and Sam Sweeney