An intensifying search-and-rescue mission was underway Monday after a submersible used for tourist expeditions to view the wreck of the Titanic went missing Sunday with five people aboard in the North Atlantic, according to the U.S. Coast Guard.

The U.S. Coast Guard Northeast said on Twitter that it was searching for a 21-foot submersible from the Canadian research vessel Polar Prince that lost contact about 900 miles east of Cape Cod, Mass. The vessel “submerged Sunday morning, and the crew of the Polar Prince lost contact with them approximately 1 hour and 45 minutes into the vessel’s dive,” the Coast Guard wrote on Twitter.

The BBC reported that the Boston Coast Guard is conducting a search-and-rescue mission for the sub off the coast of Newfoundland, an eastern province in Canada.

Officials are holding a press conference Monday at 1:30 p.m. Pacific.

The Guardian reported that the sub is operated by OceanGate Expeditions, a company that offers visits to the Titanic wreck.

OceanGate confirmed its vessel was missing and posted a statement to Twitter on Monday afternoon. “We are exploring and mobilizing all options to bring the crew back safely,” the company said. “Our entire focus is on the crew members in the submersible and their families.”

The Joint Rescue Coordination Centre in Halifax, Nova Scotia, received a call at 9:13 p.m. Sunday from the maritime rescue coordination center in Boston requesting “assistance for the search of the overdue research sub,” said Lt. Commander Len Hickey. The sub had lost contact with its surface vessel, Hickey said.

The center provided a fixed-wing aircraft and a Canadian coast guard vessel to aid in the search.

Hickey was unable to provide more information and referred additional questions to the Boston Coast Guard, which is leading the effort.

David Concannon, an advisor to the company, said OceanGate lost contact with the sub Sunday morning. It had a 96-hour oxygen supply, he said in an email to the Associated Press on Monday afternoon. Concannon was supposed to be on the dive but could not go because of another client matter. He said officials are working to get a remotely operated vehicle that can reach a depth of about 20,000 feet to the site as soon as possible.

The Titanic, which sank in 1912, is about 13,000 feet at the bottom of the Atlantic Ocean nearly 400 nautical miles off the Newfoundland coast.

According to OceanGate’s website, which worked intermittently Monday, a deep sea voyage to view the Titanic wreck was underway.

Representatives for the U.S. Coast Guard and the Boston Coast Guard did not immediately return repeated requests for comment. Representatives for OceanGate Expeditions also did not immediately return repeated requests for comment.

According to its website, OceanGate is a privately owned company in Everett, Wash., established in 2009 that operates a trio of five‑person submersibles for “site survey, scientific research, film production and exploration travel.” Its vessels can reach about 13,123 feet deep, the company said.

The company offers an eight-day, seven-night voyage to the Titanic wreck, according to its website. The cost runs about $250,000, according to the site.

“Become one of the few to see the Titanic with your own eyes,” the company says on its Titanic expeditions page.

The trip sets off from and returns to the city of St. John’s in Newfoundland, Canada, and takes “intrepid travelers” onboard a submersible called the Titan to explore the site of the Titanic wreck. Dive expeditions can begin as early as Day 3, according to the website. The Titan carries up to five people, the website said.

According to the website, no previous dive experience is necessary but details some physical requirements, including being able to board small boats in rough seas and sit for long periods of time. Explorers must also be at least 18 or older, the website said.

Those embarking on the expedition receive a vessel orientation and safety briefing after boarding the ship, the company said on its website.

The Titanic was a British luxury liner that made its maiden voyage from Southampton, England, on April 10, 1912, bound for New York with 2,227 passengers and crew aboard. But the vessel, then the largest in the world, rammed into an iceberg and sank to the bottom of the Atlantic in the early morning hours of April 15, killing more than 1,500 people.

In September 1985, an American and French team of researchers found the liner thousands of feet down.

The Associated Press contributed to this report. This is a breaking news story and will be updated as additional information becomes available.