Warnings over the safety of OceanGate's Titan submersible were repeatedly dismissed by the CEO of the company, email exchanges with a leading deep sea exploration specialist show.
In messages seen by the BBC, Rob McCallum told OceanGate CEO Stockton Rush that he was potentially putting his clients at risk and urged him to stop using the sub until it had been certified by an independent agency.
Mr Rush responded that he was "tired of industry players who try to use a safety argument to stop innovation".
The tense exchange ended after OceanGate's lawyers threatened legal action, Mr McCallum said.
"I think you are potentially placing yourself and your clients in a dangerous dynamic," he wrote to the OceanGate boss in March 2018. "In your race to Titanic you are mirroring that famous catch cry: 'She is unsinkable'".
In the messages, Mr Rush, who was among five passengers who died when the Titan experienced what officials believe was a "catastrophic implosion" on Sunday, expresses frustration with the criticism of Titan's safety measures.
"We have heard the baseless cries of 'you are going to kill someone' way too often," he wrote. "I take this as a serious personal insult."
Mr McCallum told the BBC that he repeatedly urged the company to seek certification for the Titan before using it for commercial tours. The vessel was never certified or classed.
"Until a sub is classed, tested and proven it should not be used for commercial deep dive operations," he wrote in one email.
"I implore you to take every care in your testing and sea trials and to be very, very conservative," he added. "As much as I appreciate entrepreneurship and innovation, you are potentially putting an entire industry at risk."
In his response a few days later, Mr Rush defended his business and his credentials.
He said OceanGate's "engineering focused, innovative approach... flies in the face of the submersible orthodoxy, but that is the nature of innovation".