A submersible went missing in the area of the Titanic wreck on Monday, setting off a search-and-rescue operation by the U.S. Coast Guard, the agency said.

Petty Officer Lourdes Putnam confirmed that Coast Guard officials were searching for the submersible. It was not clear how many people were on board the vessel, if any, or who was operating it, and Officer Putnam offered no further details.

At least one company has taken tourists to see the Titanic in recent years. The company, OceanGate Expeditions, said on its website on Monday that an expedition was “currently underway.” The company has offered tours of the Titanic in which guests paid $250,000 to travel to the wreckage on the seabed, more than two miles below the ocean’s surface.

The Titanic sank in 1912 on its maiden voyage from England to New York after hitting an iceberg, killing more than 1,500 people. The wreckage was found in 1985, broken into two main sections, about 400 miles off Newfoundland, in eastern Canada, and has since attracted the attention of experts and amateurs alike.

This is a developing story. Check back for updates.