Human remains are believed to have been found in the wreckage of the Titan submersible, the US Coast Guard says.

Pieces from the sub, which imploded on a deep dive to the Titanic, were unloaded in St John's, Canada, on Wednesday.

Authorities say the sub's arrival landing frame and a back cover were found among the debris.

US medical experts will officially analyze presumed remains, the coast guard said in a statement.

The agency is in the beginning phases of an investigation concerning the reasons for the Titan disaster. The Coast Guard's Marine Board of Investigation (MBI) will transport the evidence to a US port for further analysis and testing.

MBI Chair Captain Jason Neubauer said in a statement that "still a substantial amount of work to be done to understand the factors that led to the catastrophic loss of the Titan and help ensure a similar tragedy does not occur again".

"I am grateful for the coordinated international and interagency support to recover and preserve this vital evidence at extreme offshore distances and depths," Cpt Neubauer said.

All five people on board the vessel died on 18 June after it imploded about 90 minutes into a dive to view the famous 1912 shipwreck, which sits at a depth of 3,800m (12,500ft) in the North Atlantic.

The passengers were the head of OceanGate, which organized the dive, 61-year-old Stockton Rush; British explorer Hamish Harding, 58; Shahzada Dawood, 48, and his son, Suleman Dawood, 19; and French diver Paul-Henry Nargeolet, 77.

Officials were initially skeptical about the prospect of recovering any of the bodies.

"This is an incredibly unforgiving environment down there on the seafloor," Coast Guard Adm John Mauger said shortly after the loss of the vessel was confirmed.

Cpt Neubauer said at the time that investigators would take "all precautions" if they discovered human remains and that there would likely be a formal hearing including witness testimony.

So far, the coast guard says, five major pieces of the sub have been found in a large debris field near the bow of the Titanic.

The debris brought ashore on Wednesday appeared to include at least one titanium end cap, the sub's porthole with its window missing, as well as a titanium ring, landing frame, and the end equipment bay, according to BBC science correspondent Jonathan Amos.

OceanGate has been criticized for its safety practices, and former employees have raised concerns about the Titan sub, which was not subject to regulation.

In email messages seen by the BBC, Mr. Rush had previously dismissed safety worries from one expert, saying he was "tired of industry players who try to use a safety argument to stop innovation".

Another former OceanGate employee also wrote an inspection report identifying "numerous issues that posed serious safety concerns", including how the hull had been tested.

In a statement last week, OceanGate said it was "an extremely sad time for our employees who are exhausted and grieving deeply over this loss".

Rest in peace