The U.S. Coast Guard has launched a search for a missing tourist submersible that disappeared Sunday after setting out on a survey the wreck of the Titanic, told NBC Boston. It is not yet known how many people were on board the vessel when it disappeared.

OceanGate Expeditions, a private company that charters trips to explore the wreckage of the Titanic, said it was working with multiple government agencies to help rescue its crew.

“Our entire focus is on the crew members of the submarine and their families, we are deeply grateful for the extensive assistance we have received from several government agencies and deepwater companies in our efforts to re-establish contact with the submarine,” the company said in a statement.

The sub was part of an OceanGate trip to explore the iconic Titanic wreck, located 900 nautical miles east of Cape Cod, Massachusetts. However, it was not possible to return it on Sunday.

The Coast Guard coordinated a search to locate or recover the submarine and any survivors.

Considered the world’s most famous shipwreck, the Titanic was an opulent 883-foot-long ocean liner. In 1912, considered “unsinkable”, it shocked the world when it hit ice in the Atlantic Ocean and sank. More than 1,500 people died.

The wreck was discovered in 1985, about 350 miles offshore Newfoundland, Canada. The two main parts of the ship, surrounded by debris, are about 2,000 feet apart, according to a full-size scan that was released earlier this year.

The continuing fascination with the Titanic has fueled a tourism industry surrounding the ocean liner and its sinking, particularly following the release of the 1997 film of the same name directed by James Cameron.

OceanGate tours include a 10-day trip with 18 dives from a larger ship to a maximum depth of 12,800 feet. According to the company, scientific research and observations are also carried out on the trips.

This is a developing story. Watch for updates.