Titanic tourist sub missing for third day as search teams race clock

The 21-foot-long Titan was built to stay underwater for 96 hours, according to its specifications - giving the five people aboard until Thursday morning before air runs out. One pilot and four passengers were inside the miniature sub early on Sunday when it lost communication with a parent ship on the surface about an hour and 45 minutes into its two-hour dive.

As Canadian and U.S. authorities stepped up the search, previous questions about the safety design and development of the submersible by its owner, U.S.-based OceanGate Expeditions, came to light.

The wreck of the Titanic, a British ocean liner that struck an iceberg and sank on its maiden voyage in April 1912, lies about 900 miles (1,450 km) east of Cape Cod, Massachusetts, and 400 miles (644 km) south of St. John's, Newfoundland.

U.S. and Canadian aircraft have searched more than 7,600 square miles of open sea, an area larger than the state of Connecticut, U.S. Coast Guard Captain Jamie Frederick told reporters at a press conference on Tuesday.

The Canadian military has dropped sonar buoys to listen for any sounds that might come from the Titan, and a commercial vessel with a remote-controlled deepwater submersible was also searching near the site, Frederick said.

Separately, a French research ship carrying its own deep-sea diving robot vessel was dispatched to the search area at the request of the U.S. Navy and was expected to arrive Wednesday night local time, the Ifremer research institute said.

Those aboard Titan for a tourist expedition that costs $250,000 per person included British billionaire Hamish Harding, 58, and Pakistani-born businessman Shahzada Dawood, 48, with his 19-year-old son Suleman, who are both British citizens.

French explorer Paul-Henri Nargeolet, 77, and Stockton Rush, founder and CEO of OceanGate Expeditions, were also reported to be on board. Authorities have not confirmed the identity of any passenger.

Rescuers face significant obstacles both in finding the Titan and in saving the people aboard, according to experts.

If the submersible experienced a mid-dive emergency, the pilot would likely have released weights to float back to the surface, according to Alistair Greig, a marine engineering professor at University College London. But absent communication, locating a van-sized submersible in the vast Atlantic could prove challenging, he said.

The submersible is sealed with bolts from the outside, preventing the occupants from escaping without assistance even if it surfaces.