The US Securities and Exchange Commission is investigating whether recent stock sales by Elon Musk, the CEO of Tesla, and his brother Kimbal Musk “violated insider trading rules”, the Wall Street Journal reported on Thursday, citing people familiar with the matter. According to the report, the investigation began last year after Kimbal sold shares of the electric carmaker valued at $108m, a day before Musk polled Twitter users asking whether he should offload 10% of his stake in Tesla. The SEC issued a subpoena on 16 November, 10 days after Musk’s poll, seeking information related to some financial data. The investigation would escalate Musk’s battle with regulators as they scrutinize his social media posts and Tesla’s treatment of workers, including accusations of discrimination. Last week, Tesla and Musk accused the SEC of harassing them with an “endless” and “unrelenting” investigation to punish Musk for being an outspoken critic of the government. Tesla’s shares were up about 1% in late-afternoon trading, paring losses amid a broader stock market rout. The stock has fallen about 33% since Musk began selling billions of dollars worth of shares on 8 November, few days after the poll where 58% of voters asked him to sell. The SEC, Tesla and Kimbal Musk did not immediately respond to Reuters’ requests for comment.