"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 tourist submersible that disappeared in the Atlantic Ocean on Sunday is believed to be at depths that greatly exceed the capabilities of the NATO Submarine Rescue System (NSRS), according to a spokesperson for the United Kingdom's Ministry of Defense.

"As the host nation for NATO’s multinational submarine rescue capability, we continue to monitor the incident in the North Atlantic and will guide and assist in any response activity as appropriate," the spokesperson told ABC News in a statement on Tuesday.

The U.K. has not been approached to offer assistance in the ongoing search for the deep-sea vessel off the coast of Newfoundland, Canada, according to the spokesperson.

Initial reports indicate that the depths of water involved greatly exceed that which the NSRS team can safely operate -- 610 meters for the NSRS submersible and 1,000 meters for the NSRS remotely operated vehicle, according to the spokesperson.

The NSRS is based at the home of the U.K. Royal Navy Submarine Service in HM Naval Base Clyde, the U.K. Royal Navy's headquarters in Scotland. Introduced in 2006, the tri-national capability team can respond to a stricken submarine in rescuable water which is capable of mating with the NSRS submarine rescue vehicle, according to the spokesperson.

-ABC News’ Emma Ogao