Elon Musk is due to resume testifying in his own defense on Wednesday in a defamation lawsuit filed by a British diver whom the Silicon Valley billionaire called "pedo guy" on Twitter.
Musk, the 48-year-old CEO of Tesla, apologized for the tweet during his first day of testimony in U.S. District Court in Los Angeles on Tuesday, saying he was responding to an "unprovoked" insult by plaintiff Vernon Unsworth.
Unsworth was leading a rescue of 12 boys and their soccer coach from a flooded cave in Thailand last year when, in an interview on CNN, he mocked Musk's offer of a mini-submersible as a public relations stunt. He also said the Tesla founder should "stick his submarine where it hurts."
Musk, wearing a dark suit and occasionally sipping water, started testimony with his arms tightly crossed in front of him. He clashed frequently with Unsworth lawyer L. Lin Wood, to the point where Judge Stephen Wilson warned "Let's cut the repartee. ... That's an order, not an invitation to a dance."
Musk contends that Unsworth criticized him first, and he was responding with similarly heated rhetoric and was not making a literal accusation of pedophilia.
The dispute began when Unsworth criticized Musk for his efforts to involve himself in the Thai cave rescue in July 2018.
Musk and his employees developed a mini submarine, which they billed as an escape pod that could transport the kids out of the caves. On July 8, 2018, Musk wrote in a tweet, "Mini-sub arriving in about 17 hours. Hopefully useful. If not, perhaps it will be in a future situation."
The sub was never used in the effort. After the rescue, Unsworth was asked during a CNN interview about the mini-sub and Musk. He said Musk could "stick his submarine where it hurts," and viewed Musk's efforts as "just a PR stunt."
Lashing back, Musk called the caver a "pedo guy" in public tweets, and "child rapist" in an email to a reporter at Buzzfeed. Unsworth filed the lawsuit in the U.S. in September 2018.
At one point in his testimony, Musk said: "I assume he literally didn't mean to sodomize me with a submarine. I literally didn't mean he was a pedophile."
Musk attorney Alex Spiro said in his opening statements that the case was "about insults not statements of fact." He said Musk's tweets were "not allegations of a crime," but only "joking taunting tweets in a fight between men."
Wood asked Musk how influential he thought he is.
"I'm not sure to the degree I'm influential," Musk replied. "I have asked people to take climate change seriously ... and I haven't been successful."
When Wood asked whether Musk thought he should choose his words carefully, the Tesla CEO replied, "There are a lot of things I say, and not all of them have the same quality of thought, ... not everything can be thoughtful."
Wood proceeded to review the timeline of Musk's efforts in the Thai cave rescue.
Musk said at first he thought his help would not be needed, but after a Thai Navy SEAL, Saman Kunan, died in the rescue, with a monsoon bearing down, he changed his mind. "If immediate action wasn't taken, the boys would die," Musk said. "That would be on my conscience forever. I've got to do something. We've got to try."
Before an afternoon break, Musk argued that Unsworth's comments about his involvement in the effort amounted to an "unprovoked attack." Musk also described those comments as "wrong and insulting," and said that's why he insulted Unsworth back.
Later, Musk described a letter Wood sent asking for a settlement a "shakedown letter" from a "shakedown lawyer," saying "The core truth of it is extortion."
But Musk admitted that an investigator he hired to try and bolster his claims of Unsworth's pedophilia or other criminal activity was a "professional con man."
In a surprise moment, Musk's lawyer, Spiro, asked him to describe his childhood in South Africa. Musk paused for long time before finally answering, "It wasn't good." He left the country at age 17.
—CNBC's Jane Wells and Paul McNamara and Reuters contributed to this report.