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 US media.

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.