A touristic expedition submarine used for showing tourists the wreckage of Titanic in the Atlantic Ocean has gone missing off Canada's southeastern coast on Monday.

On its website, the company said an expedition to the Titanic site was "underway."

In a statement quoted by CBS News and others, OceanGate Expeditions said, "Our entire focus is on the crew members in the submersible and their families."

The U.S. Coast Guard said there was one pilot and four passengers on board and that the vessel had the capacity to be submerged for 96 hours, but it was unclear whether it was still underwater or had surfaced and was unable to communicate.

U.S. and Canadian ships and planes have swarmed the area about 900 miles (1,450 km) east of Cape Cod, some dropping sonar buoys that can monitor to a depth of 13,000 feet (3,962 meters), U.S. Coast Guard Rear Admiral John Mauger told reporters on Monday.

"It is a remote area and it is a challenge to conduct a search in that remote area," Mauger said.

"We are deploying all available assets to make sure that we can locate the craft and rescue the people on board," he said. "Going into this evening we will continue to fly aircraft and move additional vessels."

Mauger said officials have also been reaching out to commercial vessels for help.

The private company that operates the submarine, OceanGate Expeditions, said in a statement on Monday that it was "mobilizing all options" to rescue those on board. British billionaire Hamish Harding is among the passengers, according to a social media post from a relative.

The U.S. Coast Guard said earlier on Twitter that a boat on the surface - the Polar Prince - lost contact with the submarine, called the Titan, about one hour and 45 minutes after it began diving toward the site of the Titanic's wreckage on Sunday morning.

OceanGate said, "We are 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."

Harding's stepson wrote on Facebook that Harding had "gone missing on submarine" and asked for "thoughts and prayers." The stepson subsequently removed the post, citing respect for the family's privacy.

Harding himself had posted on Facebook that he would be aboard the sub. There have been no further posts from him. The expedition headed out to sea on Friday, and the first dive was set for Sunday morning, according to Harding's post.

The company is currently operating its fifth Titanic "mission" of 2023, according to its website, which was scheduled to start last week and finish on Thursday.

The expedition, which costs $250,000 per person, starts in St. John's, Newfoundland, before heading out approximately 400 miles into the Atlantic to the wreckage site, according to OceanGate's website.

In order to visit the wreck, passengers climb inside Titan, a five-person submersible, which takes about two hours to descend to the Titanic.

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, 12,500 feet (3,800 meters) under water. It was found in 1985 and remains a source of fascination and a lure for nautical experts and underwater tourists.