The family of British a billionaire’s family have accused OceanGate of taking "too long" to raise the alarm.
The Titan submarine was reported missing to the US Coast Guard on Sunday at 5.40pm, eight hours after the Polar Prince, its launch boat, lost contact with it.
Today, debris was found by a remotely-operated vehicle on the sea floor near the Titanic.
Now the family of Hamish Harding have criticised OceanGate for waiting a "frightening" amount of time call for help.
Kathleen Cosnett, a cousin of Mr Harding, said: "It took so long for them to get going to rescue [them], it’s far too long. I would have thought three hours would be the bare minimum.
Robert Evans, Mr Harding’s godson, said the family were first told the vessel was missing late on Sunday night.
He said: "I heard the sub had lost contact through family who were in contact with them [OceanGate] roughly when I was going to sleep [at around 11pm].
"At that time we as a family started scrambling to spread the word and get help.
"And we did call the UK Coastguard and Foreign [Commonwealth and Development] Office. We called them very early on", he told The Telegraph.
Another two British passengers, Shahzada Dawood, 48, a wealthy and prominent British-Pakistani businessman and his son, Suleman, 19, are also missing on the submarine.
Also aboard are the veteran French diver Paul Henri Nargeolet, 77, and Stockton Rush, 61, the founder of the vessel’s US-based operating company OceanGate.
Debris found by a remotely-operated vehicle on the seafloor near the Titanic and the findings are being 'evaluated' by experts.
The discovery comes hours after the estimated oxygen supplies would have run out.
It is unclear whether officials believe the debris is connected to the Titan vessel.
A spokesperson for the USCG wrote: "A debris field was discovered within the search area by an ROV near the Titanic. Experts within the unified command are evaluating the information."
A conference has been arranged for 3pm EST (8pm UK time) to “discuss the findings”.
The findings are being 'evaluated' by experts.
Asked if he believes the oxygen on board will have run out by now, Mr Mauger told Sky News: "We continue to keep the crew members and the families in our thoughts as we proceed with this search and rescue while we're cognisant of the time and we've factored in a lot of data and information into the search.
"This is still an active search and rescue at this point and we're using the equipment that we have on the bottom right now, the remote operated vehicles to expand our search capability, and then also to provide rescue capability as well."
The US Coast Guard said it was deploying two remotely operated vehicles, which is now on the seabed.
The French government-backed vessel is about to deploy its own ROV, Victor 6000, into the ocean, the Coast Guard said.
The Horizon Artic ship arrived at the site of Titanic wreckage last night after travelling 400 miles overnight across the Atlantic Ocean from St John's port on the eastern Canadian coast to help join the frantic search.
Rear Admiral John Mauger revealed yesterday it was unclear if banging noises had come from the submersible, as the noise was the "focus" of the mission.
Mr Mauger revealed they "don't know the source of that noise" but have reported the data with the US Navy. However, an initial attempt to locate the submarine had “yielded negative results.”
The US Coastguard will hold another press conference at 8pm tonight UK time.
