Tesla founder Elon Musk and his brother Kimbal Musk are under investigation on suspicions that they violated insider trading rules last year, The Wall Street Journal reported Thursday.

According to the Journal's sources, the investigation began in 2021 after Kimbal Musk sold shares of Tesla valued at $108 million, one day before Elon Musk polled Twitter users asking whether he should unload 10% of his stake in Tesla and pledging to abide by the vote’s results. At the time, Elon Musk said he made the poll amidst a debate in Congress on raising taxes on the wealthy, something he opposes.

Before the tweet went out, Kimbal Musk sold about 88,500 shares on the day before the poll. In the wake of the poll, Tesla's shares fell by about 15.4% after 58% of Twitter voters said Elon Musk should sell. According to filings with the SEC, Kimbal Musk sold shares that totaled about $109 million in value.

This investigation is the latest episode in a growing saga of confrontations between Elon Musk and the U.S. government’s top securities regulator.

On Feb. 17, lawyers for Tesla filed a lawsuit against the SEC that accused the agency of trying to muzzle Elon Musk over his criticisms of the Biden administration. However, the complaints arise from a settlement struck with the SEC in 2018 under the Trump administration. The deal required the distribution of $40 million to Tesla shareholders, who claim a 2018 Twitter post by Elon Musk about returning Tesla to being a private company cost them millions.

As part of the deal, the company reached a settlement that required certain tweets from Elon Musk to receive “preclearance” before they are posted.

This did not stop Elon Musk from tweeting on May 1, 2020, that he felt “Tesla’s stock price is too high imo,” using an abbreviation for “in my opinion.” Tesla’s shares sank after the tweet, but a federal judge prevented a lawsuit from going forward.