Presumed human remains have been found within 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.

Officials say the sub's landing frame and a rear cover were found among the debris.

US medical professionals will conduct a formal analysis of presumed remains, the coast guard said in a statement.

The agency is in the early stages of an investigation into the causes of the 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 Capt Jason Neubauer said in a statement that there was "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," Capt 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 organised 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 sceptical 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 the probe would likely include a formal hearing with 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.

The recovery mission was led by Canadian ship Horizon Arctic, which carried a remotely operated vehicle operated by Pelagic Research Services. The company said in a statement earlier Wednesday that its team had completed off-shore operations and was returning to base.

OceanGate has been criticised 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 which identified "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".