"You published through three different tweets to your twenty-two million followers that Mr. Unsworth engages in the sexual exploitation of Thai children", Unsworth's attorney L. Lin Woodwrote in an August 6 letter first published on Wednesday by Buzzfeed News.
While the latest communiqués avoided any reference to the troubled auto company, his August 7 "funding secured" tweet indicating he had raised billions needed to take Tesla private at US$420 a share triggered the US Securities and Exchange Commission to intensify a pre-existing investigation into firm's public pronouncements on vehicle production forecasts and two class-action investor lawsuits.
Musk is known for being unusually candid on Twitter compared to other CEOs, but he has become increasingly combative on the site this year, lashing out at critics and reporters and spurring questions about his judgment when using the site.
The agency is reportedly investigating whether Musk was attempting to hurt the company's short-sellers when he published the tweet.
Mr Musk has more than 22 million Twitter followers and frequently interacts with users who refer to him on the platform.
Musk steered clear of making any assertions about Tesla, a key distinction from his posts that are said to be under investigation by the Securities and Exchange Commission. Musk later issued a statement that Tesla would remain a publicly traded company. "You did so without any basis".
"Nonetheless, his actions against me do not justify my actions against him, and for that I apologize to Mr. Unsworth and to the companies I represent as leader", Musk tweeted.
To keep tabs on Tesla inventory, @Latrilife monitors California lots where he said Tesla cars are kept before being shipped to distribution centers and posts the footage on Twitter.
But Musk's decision to continue to respond to critics on Twitter indicates he won't change easily, said Charles Elson, a University of DE finance professor who specializes in corporate governance.