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.