Elon Musk on Monday warned that he might walk away from his $44 billion offer to acquire Twitter Inc if the social media network failed to provide data on spam and fake accounts.

In a letter to Twitter, the billionaire reiterated his request for details on bot accounts and said he reserved all rights to terminate the merger as the company was in a “clear material breach” of its obligations by not providing him with the information.

Twitter shares were down 5.5% at $38.13 and were trading at a steep discount to Musk’s offer of $54.20 per share, suggesting that investors did not expect the deal would close at the agreed price.

Twitter did not immediately respond to a request for comment.

This is the first time Musk has threatened to walk away from the deal in writing as opposed to airing it on Twitter’s social media platform.

“Musk believes Twitter is transparently refusing to comply with its obligations under the merger agreement, which is causing further suspicion that the company is withholding the requested data,” according to the letter.

Twitter had previously downplayed Musk’s warning that the deal was “on hold”, arguing the data would help him prepare for his ownership of Twitter and that it was not meant to carry out due diligence and reopen negotiations.

Earlier in May, Musk said he would put the deal “temporarily on hold”, while he waits for the social media company to provide data on the proportion of its fake accounts.

In response, Twitter Chief Executive Parag Agrawal had said the most advanced spam campaigns used combinations of humans and automation and that he did not believe the calculations could be performed externally, because it required both public and private information that Twitter cannot share.

Musk said in his letter that he needed the data to conduct his own analysis of Twitter users and did not believe in the company’s “lax testing methodologies.”

“He is trying to walk away from the Twitter deal, this is the first shot across the bow,” Wedbush analyst Dan Ives said.

A self proclaimed free-speech absolutist, Musk, who owns 9.6% of Twitter and is its second-largest shareholder, has said one of his priorities will be to remove “spam bots” from the platform.

The Tesla Inc boss has also secured financing for the deal and has pulled in top shareholders, including Saudi Arabian investor Prince Alwaleed bin Talal and Sequoia Capital.