As billionaire investor Elon Musk continues to face accusations of labor law violations after he tweeted an apparent threat at workers attempting to unionize his California Tesla factory, Rep. Keith Ellison (D-Minn.) sent a letter on Friday demanding that Musk answer for his "anti-union" tweets as well as "a series of anti-worker incidents" that have reportedly taken place at Tesla's electric car factory over the past year.

"I hope that Mr. Musk realizes that retaliation he may take against his workers for trying to form a union isn't just morally wrong—it's against the law."
—Rep. Keith Ellison

"Workers at a company with as many workplace health and safety issues as Tesla could benefit greatly from forming a union," Ellison wrote in his letter (pdf) to Musk, citing a recent investigation by Reveal that uncovered pervasive workplace injuries at Tesla that often went unreported. "I am concerned by Mr. Musk's tweets, which appear to be in violation of the Fair Labor Standards Act."

Calling Reveal's reporting and Musk's tweets "troubling," Ellison demanded answers to four questions:

• Will you refrain in the future from threatening your employees with retaliation for exercising federally protected rights, including but not limited to their Section 7 rights to join a union and bargain collectively?
• Will you refrain in the future from stating or implying that employees will lose job-related benefits if they choose union representation?
• Will you refrain in the future from adverse action against an employee for choosing to join or support a union?
• Regarding the health and safety record at the Tesla plant, will you explain the thirteen injuries added to Tesla’s official injury log for 2017 that were not reported initially?

"I hope that Mr. Musk realizes that retaliation he may take against his workers for trying to form a union isn't just morally wrong—it's against the law," Ellison noted in a tweet on Friday. "Considering the high rate of workplace injury at Mr. Musk's Tesla plant, it seems very sensible to me that workers there would want the benefits of a union."