Rescuers were in a race against time to find a missing submersible on Tuesday, two days after it lost communication while taking wealthy tourists, including two Pakistanis, to see the wreckage of the Titanic in deep waters off Canada’s coast.

The 21-foot craft, operated by OceanGate Expeditions, began its descent on Sunday but lost contact with the surface less than two hours later, according to authorities.

One pilot and four passengers were on board the submersible that disappeared on Sunday. The operating company said it had the capacity to stay underwater for up to 96 hours — giving those aboard until early on Thursday before the air ran out.

US Coast Guard Rear Admiral John Mauger in television interviews on Tuesday said rescuers continued their efforts overnight and were expanding their search into deeper waters, telling NBC News that authorities were prioritising the area where the vessel was operating.

Those aboard the submersible, the highlight of a tourist expedition that costs $250,000 per person, included British billionaire Hamish Harding.

The 77-year-old French explorer Paul-Henri Nargeolet and Stockton Rush, founder and CEO of the vessel’s US-based operating company OceanGate, were also reported to be on board. Authorities have not confirmed the identity of any passenger.

However, family of Pakistan businessman Shahzada Dawood has confirmed that he was aboard alongwith and his son, Suleman.

US and Canadian ships and planes have been sweeping the area about 1,450 kilometres east of Cape Cod, some dropping sonar buoys that can monitor to a depth of 3,962 metres.

Rescuers face significant challenges both in finding the Titan and in saving the people aboard, according to experts.

France’s maritime ministry has said the country’s oceanographic institute is sending its vessel Atalante, equipped with a deep-sea underwater robot, to the north Atlantic to help find the submersible.

The Atalante, currently on a mission for the French Research Institute for Exploitation of the Sea, should arrive in the area on Wednesday around 1800 GMT, with experts urgently dispatched from the French port of Toulon to operate the robot for its dive, the ministry added.

Pakistani businessman, son on board

Businessman Shahzada Dawood and his son, Suleman, were also on board, their family said in a statement. The statement said the father and son had planned the journey to visit the remnants of the Titanic shipwreck.

“We are very grateful for the concern being shown by our colleagues and friends and would like to request everyone to pray for their safety,” the Dawood family said on Tuesday.

Shahzada is the vice chairman of one of Pakistan’s largest conglomerates, Engro Corporation, which has investments in fertilisers, vehicle manufacturing, energy and digital technologies.

According to the website of SETI, the California-based research institute of which he is a trustee, he lives in Britain with his wife and two children.

In a statement, Engro corporation said that all it knew was that contact was lost with the submersible craft. “There is limited information available beyond this that we know, and we humbly request that speculation and theorisation is avoided,” the company said.

“We, at Engro, remain in prayer for their swift and safe return, and will share updates we may have as and when they come,” it added.

“It is a challenge to conduct a search in that remote area, but we are deploying all available assets to make sure that we can locate the craft and rescue the people on board,” US Coast Guard Rear Admiral John Mauger told reporters in Boston, where he was overseeing the operation.

OceanGate Expeditions’ spokesperson told AFP late on Monday that “for some time, we have been unable to establish communications with one of our submersible exploration vehicles which is currently visiting the wreck site of the Titanic”.

“Our entire focus is on the well-being of the crew and every step possible is being taken to bring the five crew members back safely,” the statement added.

The company uses a submersible named Titan for its dives to a maximum depth of 4,000 metres.

‘Clock is ticking’

The Titanic hit an iceberg and sank in 1912 during its maiden voyage from England to New York with 2,224 passengers and crew on board. More than 1,500 people died in the tragedy.

The wreckage is in two main pieces 400 miles off the coast of Newfoundland, Canada, some 13,000 feet underwater. It was found in 1985 and remains a source of fascination and a lure for nautical experts and underwater tourists.

Without having studied the craft itself, Alistair Greig, professor of marine engineering at University College London, suggested two possible theories based on images of the vessel published by the press.

He said if it had an electrical or communications problem, it could have surfaced and remained floating, “waiting to be found”.

“Another scenario is the pressure hull was compromised — a leak,” he said in a statement. “Then the prognosis is not good”.

While the submersible may still be intact during its dive, there are very few vessels able to go to the depth to which the Titan might have travelled.

OceanGate said in its statement it was “deeply thankful for the extensive assistance we have received from several government agencies and deep sea companies in our efforts to reestablish contact with the submersible”.

“The clock is ticking, and any submariner/submersible deep divers know how unforgiving the Abyssal domain is: going undersea is as, if not more, challenging than going into space from an engineering perspective,” said University of Adelaide associate professor Eric Fusil in a statement.