Canada's Transportation Safety Board says it is launching an investigation after the implosion of the Titan submersible left all five passengers dead
In the US, a separate probe will be launched involving the American counterpart and US Coast Guard
Tributes have been paid to Hamish Harding who would have turned 59 on Saturday, Shahzada Dawood, 48, and his son Suleman Dawood, 19, Paul-Henri Nargeolet, 77, and Stockton Rush, 61
Emails seen by the BBC show that concerns from an expert over the safety of the Titan sub were dismissed by OceanGate CEO Rush in 2018
Other industry experts have also raised questioned following the "catastrophic implosion"
But an OceanGate investor who has made the trip down to the Titanic told the BBC that the idea Rush had done anything wrong was "disingenuous"
It's emerged the US Navy originally detected “an acoustic anomaly consistent with an implosion” shortly after the Titan lost contact on Sunday
Titanic expert: Passengers subs must be 'super safe'
We've recently heard from Tim Maltin, a Titanic expert.
He told the BBC that he believes tourism dives to the Titanic wreck should be suspended "until lessons are learned".
"We should suspend tourist deep sea diving, especially to the Titanic," he says.
"They need to be certified to greater depths than they will take them to."
Pakistan's foreign ministry has sent its "deepest condolences" to the family of British-Pakistani father and son, Shahzada and Suleman Dawood, who are two of the five crew members now known to have died on the Titan submersible.
Shahzada, 48, lived with his wife, Christine, and other child, Alina, in Surbiton, south-west London. The family were spending a month in Canada prior to the dive.
His son Suleman, 19, was a student studying at the University of Strathclyde in Glasgow.
Earlier this week a family statement described Suleman as a "big fan of science fiction literature and learning new things", and having an interest in Rubik's cubes and playing volleyball.
You can read more about him here.
Image source, Engro Corporation/DH GroupMore from David Mearns now, who tells the BBC about two of his friends - shortly after learning of their deaths.
"Hamish Harding was a terrific character," Mearns says.
Mearns says he never knew he was a billionaire. "He was gregarious."
When they first met at a dinner, by the end of it, they were planning an expedition together with their families to climb Africa's highest peak, Mount Kilimanjaro.
Mearns says Harding pushed the boundaries and did "remarkable" things in his life.
Quote MessagePersonally I’ve lost two friends in the most horrific way
David Mearns, Friend to Hamish Harding and Paul-Henry Nargeolet
As for Paul-Henry Nargeolet, Mearns says he is "almost a legend really" in terms of what he’s done in his career and his contributions to this field.
He had deep knowledge of the Titanic, Mearns says.
If he had been the one in the submersible, Mearns says he would have wanted Nargeolet responsible for the search and recovery as not many people had his skills and experience in addition to being so calm and composed.
To have lost two fantastic characters is a huge setback to our industry, Mearns says.
Image source, LOTUS EYES PHOTOGRAPHY/JOEL SAGET/AFPPaul-Henry Nargeolet (left) and Hamish Harding (right)
Titan sub: 'The only solace is that they didn't suffer for days' - David Mearns
"Well it’s the worst possible outcome," dive search expert and a friend of two of the five Titan crew, David Mearns, tells the BBC following the announcement that the five have died.
"Frankly it’s what I feared when I first got the call," he says.
He explains that those in the deep sea community understand the risks of an expedition like this, but when you get news like this it hits you in the pit of your stomach - particularly when it’s a friend.
"To hear that a father and son were doing this dive on Father's Day is also gut wrenching, isn’t it?"
He says the only solace is that they didn’t suffer for four days but the outcome is the same.
Mearns stresses how unprecedented this is. Governments will have to talk about how to proceed in what should be a forensic investigation, he says.
UK Foreign Secretary James Cleverly has paid tribute to the five Titan passengers who are now believed to have died.
"Tragic news that those on the Titan submersible, including three British citizens, have been lost following an international search operation," he tweets, external.
"The UK government is closely supporting the families affected and expresses our deepest condolences."
Here's what we learned from the US Coast Guard's press conference:
Pallab Ghosh
Science correspondent, BBC News
It is unclear at this stage which agency will lead the investigation as there is no protocol for such incidents with a submersible.
Rear Adm Mauger said that it is particularly complex because the incident took place in a remote part of the ocean, involving people of many different nationalities.
But having played a leading role in the operation so far, the US Coast Guard is likely to continue to play an important role.
Pallab Ghosh
Science correspondent, BBC News
Now that they have found the debris, there will be an investigation into what happened. It is important to await the results and conclusion of the experts to discover what happened.
According to Rear Adm John Mauger, the pattern of the debris is consistent with “a catastrophic implosion".
That is because there were two patches of debris: one containing the Titan’s tail cone and the other its landing frame – suggesting that the vessel blew apart.
A spokesperson for the US Coast Guard says there are no future press conferences planned following the announcement of today's tragic news.
With that, the press conference comes to an end.
US Coast Guard confirms Titan sub pressure loss
The debris field is consistent with a catastrophic implosion of the vessel, Rear Adm Mauger says.
Throughout the search, the coastguard had listening devices in the water, but did not detect any catastrophic failures, he reiterated.
Rear Adm Mauger says right now there are nine vessels on the scene, as the search for more details continues.
"We have medical personnel on the scene, we have other technicians on scene, and we will begin to demobilise personnel and vessels from the scene over the course of the next 24 hours," he says.
"This is an incredibly difficult and dangerous environment to work in."
There doesn't appear to be any connection between the noises and the location on the sea floor, Rear Adm Mauger says.
It is too early to tell what the timing of the implosion was, Rear Adm Mauger tells reporters.
He said the US Coat Guard had sonar buoys out in the water for 72 hours. Since they had been in place, they had not detected "any catastrophic events," he adds.
Image source, ReutersThe US Coast Guard says their thoughts are with the families and making sure they have an understanding as best as they can provide on what happened and to begin to find some closure.
In terms of the large process, they are going to continue to investigate the site of the debris field.
There’s a lot of questions about how, why and when this happened.
Those are questions that we will collect now while governments meet and discuss an investigation.
The site of the fatal accident is believed to be 1600ft (487m) off the bow of the Titanic wreck.
It's in an area where there is not any debris of the Titanic.
Rear Adm Mauger says he cannot confirm whether the US Coast Guard will be able to recover the bodies of the five passengers on board the Titan sub.
"This is an incredibly unforgiving environment," he adds.
Titan sub: US Navy official details what Titan debris was found
Officials say five major pieces were detected amid the debris around the Titanic site.
Among them, a nose cone, outside the pressure hull, and a larger debris field.
"On behalf of the US Coast Guard and the entire unified command, I offer my deepest condolences to the families," Rear Adm Mauger says.
"I can only imagine what this has been like for them," he adds.
"Additionally, we have been in close contact with the French and British consuls general to ensure that they are fully apprised and that their concerns are being fully addressed.
"The outpouring of support during this highly complex search operation has been robust and immensely appreciated."
Image source, Dawood Family/Lotus Eye Photography/ReutersThe US Coast Guard says they are still working to develop the details for the timeline involved.
It's an "incredibly complex environment" on the sea floor, 2 miles beneath the surface, Rear Adm Mauger says.
US Coast Guard officials say the remote operating vehicles (ROVs) operating on the sea floor around the Titanic will remain on the scene.