The U.S. Coast Guard said Thursday that an underwater vessel has located a debris field near the Titanic in the search for a missing submersible with five people aboard, a potential breakthrough in an increasingly urgent around-the-clock effort.

The Coast Guard’s post on Twitter gave no details, such as whether officials believe the debris is connected to the Titan, which was on an expedition to view the wreckage of the Titanic. The search passed the critical 96-hour mark Thursday when breathable air could have run out.

The Titan was estimated to have about a four-day supply of breathable air when it launched Sunday morning in the North Atlantic — but experts have emphasized that was an imprecise approximation to begin with and could be extended if passengers have taken measures to conserve breathable air. And it’s not known if they survived since the sub’s disappearance.

Rescuers have rushed ships, planes and other equipment to the site of the disappearance. On Thursday, the U.S. Coast Guard said an undersea robot sent by a Canadian ship had reached the sea floor, while a French research institute said a deep-diving robot with cameras, lights and arms also joined the operation.