"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

The United States Coast Guard commander leading the search for a missing submersible off the coast of Newfoundland, Canada, said Tuesday morning that crews in multiple aircraft have flown over an area of the Atlantic Ocean "roughly about the size of Connecticut" while "looking for any signs of surfacing."

"As we continue on with the search, we're expanding our capabilities to be able to search under the water as well," Rear Adm. John Mauger, commander of the First Coast Guard District, told ABC News' Robin Roberts during an interview on "Good Morning America."

A commercial vessel with remotely operated vehicles is now on scene that will allow rescuers to search underwater, according to Mauger.

"This is a complex case," he added. "The Coast Guard doesn't have all the resources to be able to affect this kind of rescue, although this is an area that's within our search zone."

In many cases, Mauger said, the Coast Guard's role is to coordinate all of the assets and technical expertise that can be used in a search, in addition to operating aircraft or ships when necessary.

"In this particular case, we've established a unified command with the United States Navy, with the Canadian Armed Forces, with the Canadian Coast Guard and with the private operator OceanGate Expeditions to make sure that we understand what's needed and deploy all available equipment to the scene that could be used to locate this submersible, whether it's on the surface or down beneath the surface," he said.

Crews have been working "around the clock" to locate the deep-sea vessel since it lost contact with its operator on Sunday morning, according to Mauger.

In the last 24 hours, a Canadian aircraft has been dropping sonar buoys into the water that can pick up sound the submersible may be emitting. Vessels that have the capability to listen with their own sonar equipment are also on scene, according to Mauger.

"If they are making sound, that's certainly one of the ways that we're going to use to locate them," he said.