Tesla Inc. (TSLA - Get Report) extended declines Friday amid renewed focus on CEO Elon Musk's use of Twitter, which had sparked a Securities and Exchange Commission probe that ultimately led to $40 million in fines and could force him from the board of the carmaker he founded.

Musk appeared to Mock the SEC, calling it the "Shortseller Enrichment Commission", late Thursday after a report from Fox Business Network suggested the regulator was looking into possible mis-statements from the carmaker linked to production and profitability of its flagship Model 3 sedan. The Tweets came just hours after U.S. District Judge Alison Nathan asked both Musk and the SEC to justify their $40 million weekend settlement over Musk's infamous 'take private' Tweet from August 7.

Tesla shares were marked 3.3% lower at the opening bell and changing hands at $272.55 each, a move that extends Tesla's year-to-date decline past 12% and values the Palo Alto, Calif.-based carmaker at around $46.2 billion.

Musk's SEC-mocking Tweet wasn't his only social media missive of the evening, however, with the billionaire pressing the case that short-selling of stock should be illegal -- while hitting out at the investment giant BlackRock (BLK - Get Report) -- arguing there is "no rational basis for a long holder to lend their stock to shorts, as it dilutes the shareholder base & gives the short a strong incentive to attack the company by whatever means possible, including regulators."

Where this breaks down is in passive index funds, which constitute most of the market. The holders of those funds, mostly small investors & retirement funds, don't realize that their stocks are being lent to short sellers, diminishing their true equity return.

— Elon Musk (@elonmusk) October 5, 2018

The Tweets appear to challenge at least one portion of the proposed SEC settlement, which deems that Musk's social media activity, as well as his communication with investors, is subject to oversight from the Tesla board.

"We think it's very unwise -- he's already on thin ice with the SEC," commented Garrett Nelson, an analyst at CFRA Research, about Thursday's apparent jab from Musk.

Nelson said that the tweet itself is unlikely to result in any additional action from the SEC, but noted that the recent settlement isn't Tesla or its CEO's only entanglement with the agency: The SEC is also looking into whether Tesla misled investors in reporting the extent of its Model 3 production issues, a probe that predated the go-private fiasco.