Elon Musk has made an offer to buy social media giant Twitter (NYSE: TWTR) for about $43 billion.

Musk made the announcement in a regulatory filing early Thursday, just a few weeks after amassing a 9.2% passive stake in Twitter to become its largest shareholder.

Musk, the world’s wealthiest person with a fortune estimated at $259 billion, disclosed in an amended 13D filing with the U.S. Securities and Exchange Commission on Thursday that he would buy the remaining shares of Twitter that he doesn’t already own for $54.20 apiece in cash.

According to the filing, the offer represents a 38% premium over Twitter’s share price on the day before his stake was made public and a 54% premium over the stock’s closing price on Jan. 28, the day before he began accumulating shares in the company.

The Tesla (NASDAQ: TSLA) founder and CEO called the bid his “best and final offer,” but he did not provide details on financing.

“I invested in Twitter as I believe in its potential to be the platform for free speech around the globe, and I believe free speech is a societal imperative for a functioning democracy,” Musk said in a letter to Twitter chairman Bret Taylor, according to the filing.

“However, since making my investment I now realize the company will neither thrive nor serve this societal imperative in its current form. Twitter needs to be transformed as a private company,” he added.

“Twitter has extraordinary potential. I will unlock it,” Musk declared.

He also warned that he might offload his stake in the company if his takeover offer was turned down.

“If [the offer] is not accepted, I would need to reconsider my position as a shareholder,” he told Taylor. “I am not playing the back-and-forth game.”

Twitter responded to the offer saying in a press release that it has received “an unsolicited, non-binding proposal from Elon Musk to acquire all of the company’s outstanding common stock.”

The company said it would carefully review the proposal to determine the course of action that it believes is in the best interest of the company and all Twitter stockholders.

Musk’s offer to take over Twitter is just the latest twist between him and the company. He recently disclosed in regulatory filings that he had been buying shares in almost daily batches from January 31.

In total, Musk accumulated 73.5 million Twitter shares, which gave him a stake of 9.2% worth about $2.9 billion.

After disclosing the stake, Twitter offered Musk an opportunity to join its board of directors, but he turned it down, sparking rumors that he could go hostile.

Joining the board would have prevented him from owning a stake of more than 14.9% in the company.

Musk has been a high-profile Twitter critic in recent months for its content-moderation practices. In March, he told his 81 million followers of the platform that he was giving “serious thought” to building a social media site.

“Given that Twitter serves as the de facto public town square, failing to adhere to free speech principles fundamentally undermines democracy,” Musk tweeted last month. “What should be done?”

Last week, he hosted a poll asking Twitter users if they would like an edit button to rewrite posts after posting. Twitter later announced it was working on such a feature. Musk also proposed changes to the Twitter Blue premium service, including a price cut, removing ads, and the option to pay in dogecoin.

Earlier this week, a group of former Twitter shareholders filed a class-action lawsuit against him, claiming he failed to properly disclose his stake in the company.

The shareholders claim they missed out on the recent rally in Twitter shares because Musk made his disclosure too late.

Under U.S. securities laws, investors who buy a stake of more than 5% in a company are required to reveal the purchase within 10 days. Musk disclosed his stake on April 4, well past the March 24 deadline.

Musk’s offer of $54.20 a share is hilarious as it includes a “420” reference to pot smoking. In 2018, he landed in trouble with the SEC and lost his position as chairman at Tesla due to his infamous “considering taking Tesla private at $420” tweet.

Twitter posted revenue of $5.08 billion at the end of last year, a 37% jump from the previous year. Its daily active users, meanwhile, increased 13% to about 217 million.

Dan Ives, analyst at investment firm Wedbush Securities, said in a note addressed to investors on Thursday that “the next step will be Twitter’s Board officially reviewing the Musk filing/letter.”

“Then it’s get-out-the-popcorn time as we expect many twists and turns in the weeks ahead as Twitter and Musk walk down this marriage path,” Ives added.

Twitter stock rose nearly 13% to $51.70 per share in the premarket trading session on Thursday. The stock is still below its 52-week high of nearly $73.