Elon Musk offered to buy Twitter for $54.20 a share in a filing published Thursday, saying the social platform needs to be transformed privately, Musk’s offer values Twitter at about $43 billion.

“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 wrote in a letter sent to Twitter Chairman Bret Taylor and disclosed in a securities filing.

Twitter shares were up about 4% Thursday morning after closing at $45.85 a share on Wednesday. Tesla’s stock plunged more than 3% on the news.

According to Musk, the social media company needs to go private because it can “neither thrive nor serve” free speech in its current state.

“As a result, I am offering to buy 100% of Twitter for $54.20 per share in cash, a 54% premium over the day before I began investing in Twitter and a 38% premium over the day before my investment was publicly announced,” he wrote. “My offer is my best and final offer and if it is not accepted, I would need to reconsider my position as a shareholder.”

Musk tapped Morgan Stanley as a financial advisor, according to the filing.

“The Twitter Board of Directors will 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,” the company said in a statement Thursday in response to the offer. CNBC’s David Faber reported on “Squawk on the Street” that Twitter’s board will meet at 10 a.m. to evaluate the bid, per people familiar.

The news comes just days after Twitter CEO Parag Agrawal warned investors of “distractions ahead.”

Shares of Twitter surged dramatically in recent weeks amid the news from Musk, but are up 6% this year and 18.5% since the start of the month.

Here is the letter Musk sent as disclosed in a securities filing:

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.

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.

As a result, I am offering to buy 100% of Twitter for $54.20 per share in cash, a 54% premium over the day before I began investing in Twitter, and a 38% premium over the day before my investment was publicly announced. My offer is my best and final offer and if it is not accepted, I would need to reconsider my position as a shareholder.

Twitter has extraordinary potential. I will unlock it.

Elon Musk