In an email to BuzzFeed News, the Tesla chief executive launched a third attack on cave diver Vernon Unsworth, who called Musk's idea to rescue the trapped boys and their coach with a miniature submarine a "PR stunt."
Musk accused Unsworth in the email of being a "child rapist," in the strongest words the billionaire has had for the explorer since referring to him as a "pedo guy" in July. Last month, Musk tweeted that it was "strange" that Unsworth had not sued him for the July comments.
Tesla shares were down 1.1 percent in premarket trading Wednesday.
Representatives for Musk and three of his companies — Tesla, SpaceX and The Boring Company — did not immediately reply to CNBC requests for comment.
Musk's initial tweet in July dampened investor sentiment in Tesla, with shares falling on the day on concerns over the executive's leadership, public image and his presence on Twitter.
Musk told BuzzFeed on Tuesday: "I suggest that you call people you know in Thailand, find out what's actually going on and stop defending child rapists, you f------ a------." Musk had said the email was off the record, but BuzzFeed said it didn't agree to the condition.
Musk alleged that Unsworth had traveled to and lived in Thailand for "30 to 40 years" and claimed — without providing evidence — that he eventually moved to Chiang Rai to find a child bride. Unsworth denied those claims, according to the news site.
Chiang Rai is the province in which the Wild Boar youth soccer team became trapped in the flooded cave. The boys and their coach were subsequently saved after weeks of efforts by local and international rescuers.
"As for this alleged threat of a lawsuit, which magically appeared when I raised the issue (nothing was sent or raised beforehand), I f------ hope he sues me," Musk said in the email to BuzzFeed.
Unsworth's attorney L. Lin Wood told CNBC Wednesday that Musk "will be" sued for his comments on the cave diver.
"Elon Musk's ongoing campaign of publishing vile and false accusations against Mr. Unsworth is reprehensible," Wood said via email.
"Musk 'hopes' to be sued and Musk deserves to be sued. He will be."
He told CNBC last week that the spelunker is preparing a case against Musk for libel damages.
Wood, an Atlanta-based attorney, has handled several high-profile libel claims, including that of Richard Jewell, the security guard who was falsely accused of bombing the Centennial Olympic Park in Atlanta in 1996.
In a letter to Musk on Aug. 6, Wood said he was preparing a civil complaint against the Tesla boss.
"You published through three different tweets to your twenty-two million followers that Mr. Unsworth engages in the sexual exploitation of Thai children, and you did so at a time when he was working to save the lives of twelve Thai children," Wood wrote at the time, adding: "You did so without any basis."
A lawsuit against Musk could be ready for filing as early as next week, CNBC understands.