A U.S. Coast Guard crew from Boston was searching on Monday for five people who were on a missing sub that explores the Titanic wreck site.

The Coast Guard was looking for the missing submersible about 900 miles off of Cape Cod.

“The @USCG is searching for a 21-foot submersible from the Canadian research vessel Polar Prince,” the U.S. Coast Guard Northeast tweeted Monday afternoon.

“The 5 person crew 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 added.

A Coast Guard Air Station Elizabeth City, North Carolina, C-130 Hercules aircraft, as well as a Canadian P8 aircraft equipped with underwater sonar capability, were searching for the missing submersible about 435 miles south of St. John’s, Newfoundland.

“A @USCG C-130 crew is searching for an overdue Canadian research submarine approximately 900 miles off #CapeCod,” the Coast Guard tweeted.

OceanGate Expeditions confirmed to the Associated Press that there was a search for its five-person sub. The maximum depth of the Titanic expedition dive is 12,800 feet, which is almost 2.5 miles on the floor of the North Atlantic Ocean.

“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,” the company said in a statement. “We are working toward the safe return of the crewmembers.”

David Concannon, an adviser to OceanGate, told the AP that the sub had a 96-hour oxygen supply.

He said officials were working to get a remotely operated vehicle that can reach a depth of about 20,000 feet to the site as soon as possible.

Action Aviation confirmed that its company chairman, U.K. businessman Hamish Harding, was one of the tourists on board. The initial group of tourists was funding the expedition by spending anywhere from $100,000 to $150,000 apiece.

Herald wire services were used in this report.