North Charleston, South Carolina - The US Coast Guard on Monday began a hearing over the implosion of the privately owned Titan submersible that killed five people on a journey to the Titanic last year.

The two-week hearing into the catastrophe will feature evidence as to what went wrong and whether physical or design failure contributed to the accident, which garnered worldwide attention.

Tony Nissen, a former engineering director at US-based OceanGate, the company which operated the submersible, testified Monday that he had been concerned by safety issues during his tenure, according to media outlets.

Nissen told the hearing, which is being held in South Carolina, that he had refused to sign off on a 2019 dive to the wreckage of the Titanic due to concerns over the Titan's hull. He was fired later that year.

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He said that the company's CEO Stockton Rush, who was among those to die in the implosion, had been unfazed after learning of a potential 2018 lightning strike on the submersible and possible hull problems.

Refusing to believe news of the damage, Rush insisted "it'll be OK," Nissen said, according to testimony from the hearing published in The New York Times.

The vessel, which was about the size of an SUV car, is believed to have imploded during its June 18, 2023 descent.

The Titan lost contact about an hour and 45 minutes after plunging into the ocean, and a days-long rescue mission which captivated the world ensued before the sub's failure was finally confirmed on June 22.

Victims are presumed to have died instantly in the disaster, which occurred under the crushing pressure of the North Atlantic at a depth of more than two miles.

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Apart from Rush, the four others on the Titan were British explorer Hamish Harding, French submarine expert Paul-Henri Nargeolet, Pakistani-British tycoon Shahzada Dawood, and his son Suleman.

A debris field was found 1,600 feet from the bow of the Titanic, which sits 400 miles off the coast of Newfoundland, Canada.

The Titanic hit an iceberg and sank in 1912 during its maiden voyage from England to New York, with 2,224 passengers and crew on board. More than 1,500 people died.