The letter comes two days after Musk tweeted an unfounded and disparaging claim about Vernon Unsworth, an expert caver who participated in the rescue mission that saved 12 boys and their soccer coach who spent almost three weeks trapped in a flooded cave in Thailand.
But asked if he would take legal action against Musk over the allegation, Unsworth said: "If it's what I think it is yes".
"Sorry pedo guy, you really did ask for it", Musk wrote.
If you're the billionaire founder of several publicly traded companies, here's an easy PR strategy: Never tweet.
Unsworth, 63, who lives in Thailand, was among the first divers on the scene in Mae Sai after the boys from the Wild Boars football team and their coach became trapped inside on 23 June.
Since then, he has been arguing with people who accuse him of shoehorning himself into the drama and imagining hypotheticals in which rescuers had not deemed his hastily built submarine unnecessary and impractical. The diver, Vern Unsworth, had criticized Musk's effort to create a small submarine to rescue the soccer team members.
Solar Shed, a British renewable-energy installer, said it would "no longer promote" Tesla-branded products and called on other solar firms to do the same, the Washington Post reported on Monday.
The entrepreneur doubled down on his claim, tweeting from his official account to more than 22 million followers: "Bet ya a signed dollar it's true".
Much like other public figures (cough, Donald Trump, cough), Musk has a reputation for tweeting first and thinking after.
What's weird about Musk's tweet is that it came on July 15, a full five days after the boys were rescued. Meanwhile, CEO of Brian Communications Brian Tierney said Musk is "not showing the kind of discipline of someone going to manufacture hundreds of thousands of cars".
"I have a lot of support from people around the world astonished by his unfounded comments", he said.
A police officer in the Chiang Rai district where Unsworth has lived for seven years, who declined to be named because he was not authorized to speak to the media, said that no charges or complaints had ever been filed against Unsworth.
The episode also seemed to repel some of Tesla's closest backers, including alternative-energy loyalists who saw in Tesla a worthy opponent to the juggernauts of the oil and gas industries.