Rescuers on Wednesday rushed more ships and vessels to the area where a submersible disappeared on its way to the Titanic wreckage site, hoping underwater sounds they detected for a second straight day might help narrow their search in an increasingly urgent mission.

Crews were scouring an area twice the size of Connecticut in waters 2 1/2 miles deep, said Captain Jamie Frederick of the First Coast Guard District, who noted that authorities are still holding out hope of saving the five passengers onboard the Titan.

“This is a search and rescue mission, 100 per cent,” he said. ”… We’ll continue to put every available asset that we have in an effort to find the Titan and the crew members.”

READ: Rescuers cling to hope in race to find Titanic sub with less than 20 hours of air left

But even those who expressed optimism warned that many obstacles remain: from pinpointing the vessel’s location, to reaching it with rescue equipment, to bringing it to the surface — assuming it’s still intact. And all that has to happen before the passengers’ oxygen supply runs out, which some have estimated might happen as early as Thursday morning.

Meanwhile, newly uncovered allegations suggest there had been significant warnings made about vessel safety during the submersible’s development.

The area of the North Atlantic where the Titan went missing on Sunday is prone to fog and stormy conditions, making it an extremely challenging environment to conduct a search-and-rescue mission, said Donald Murphy, an oceanographer who served as chief scientist of the Coast Guard’s International Ice Patrol. The lost submersible could be as deep as about 12,500 feet (3,800 metres) below the surface near the watery tomb of the Titanic.

Key to the search are camera-equipped remote-operated robots, which are designed to scan the seafloor in real-time at depths other vessels can’t reach. Two were operating in the area Wednesday and more were on the way, officials said.

READ: Underwater noises heard in desperate search for submersible missing with 5 aboard near Titanic

Frederick said while the sounds that have been detected offered a chance to narrow the search, their exact location and source hasn’t yet been determined.

“We don’t know what they are, to be frank,” he said.

Retired Navy Captain Carl Hartsfield, now the director of the Woods Hole Oceanographic Systems Laboratory, said the sounds have been described as “banging noises,” but he warned that search crews “have to put the whole picture together in context and they have to eliminate potential manmade sources other than the Titan.”

The report was encouraging to some experts because submarine crews unable to communicate with the surface are taught to bang on their submersible’s hull to be detected by sonar.

A United States (US) Navy official said during a media briefing Wednesday that a special naval salvage system that could be used to pull the Titan to the surface has arrived in St. John’s, Canada, but it is expected to take another 24 hours to prepare it for use. The Navy said in a statement that the equipment is capable of hoisting “large, bulky and heavy undersea objects such as aircraft or small vessels.”

The Titan weighs 20,000 pounds (9,071 kilograms). The Navy’s Flyaway Deep Ocean Salvage System is designed to lift up to 60,000 pounds (27,216kilograms).

The submersible had seven backup systems to return to the surface, including sandbags and lead pipes that drop off and an inflatable balloon.

Lost aboard the vessel are pilot Stockton Rush, the CEO of the company leading the expedition. His passengers are a British adventurer, two members of a Pakistani business family and a Titanic expert.

Authorities reported the 22-foot carbon-fibre vessel overdue Sunday night, setting off the search in waters about 435 miles (700 kilometres) south of St. John’s.

The submersible had a four-day oxygen supply when it put to sea around 6:00 am Sunday, according to David Concannon, an adviser to OceanGate Expeditions, which oversaw the mission.