The five passengers on board the missing Titan submersible are believed to be dead, according to the vessel's operator, OceanGate.
In a statement, the company said the five men were "true explorers who shared a distinct spirit of adventure".
The Coast Guard said that a debris field was discovered near the wreck of the Titanic earlier on Thursday morning.
The vessel disappeared on Sunday.
The men on board the sub included Stockton Rush, the 61-year-old CEO of OceanGate, as well as British-Pakistani businessman Shahzada Dawood, 48, his son Suleman, 19, and British businessman Hamish Harding, 58.
The fifth man on board, Paul-Henry Nargeolet, was a 77-year-old former French navy diver and renowned explorer.
Source: BBC