Elon Musk’s tumultuous, $44 billion bid to buy Twitter is on the verge of collapse — after the Tesla CEO sent a letter to Twitter’s board Friday saying he is terminating the acquisition.

Twitter did not immediately respond to a request for comment. It is not entirely clear whether Twitter’s board will accept the $1 billion breakup fee or if there will be a court battle over the deal.

The possible unraveling of the deal is just the latest twist in a saga between the world’s richest man and one of the most influential social media platforms. Much of the drama has played out on Twitter, with Mr. Musk — who has more than 95 million Twitter followers — lamenting that the company was failing to live up to its potential as a platform for free speech.

On Friday, shares of Twitter fell 5 percent to $36.81, well below the $54.20 that Mr. Musk had offered to pay. Shares of Tesla, meanwhile, climbed 2.5 percent to $752.29.

In a letter to the Securities and Exchange Commission, Mr. Musk said Twitter has “not complied with its contractual obligations” surrounding the deal, namely giving Mr. Musk enough information to “make an independent assessment of the prevalence of fake or spam accounts on Twitter’s platform.”

Mr. Musk’s flirtation with buying Twitter appeared to begin in late March. That’s when Twitter has said he contacted members of its board — including a co-founder, Jack Dorsey — and told them he was buying up shares of the company and was interested in joining the board, taking Twitter private, or starting a competitor. Then, on April 4, he disclosed in a regulatory filing that he had become the company’s largest shareholder after acquiring a 9 percent stake worth about $3 billion.

At first, Twitter offered Mr. Musk a seat on its board. Six days later, Twitter’s CEO, Parag Agrawal, tweeted that Mr. Musk would not be joining the board after all. His bid to buy the company came together quickly after that.

Mr. Musk had agreed to buy Twitter for $54.20 per share, inserting a “420” marijuana reference into his offer price. He sold roughly $8.5 billion worth of shares in Tesla to help fund the purchase, then strengthened his commitments of more than $7 billion from a diverse group of investors including Silicon Valley heavy hitters like Larry Ellison, an Oracle co-founder.

Inside Twitter, Mr. Musk’s offer was met with confusion and falling morale, especially after Mr. Musk publicly criticized one of Twitter’s top lawyers involved in content-moderation decisions.

As Twitter executives prepared for the deal to move forward, the company instituted a hiring freeze, halted discretionary spending, and fired two top managers. The San Francisco company has also been laying off staff, most recently part of its talent acquisition team.

By BARBARA ORTUTAY