Twitter is threatening to sue Meta over concerns about its new Threads app, according to a letter obtained by Semafor. In the letter, which is addressed to Meta CEO Mark Zuckerberg, Twitter lawyer Alex Spiro argues that Meta used Twitter’s trade secrets and intellectual property to build Threads.

Spiro, who is also Elon Musk’s personal lawyer and a partner at the Quinn Emanuel law firm, claims that Meta hired “dozens” of ex-Twitter employees to develop Threads, which wouldn’t be all that surprising given just how many people were fired following Musk’s takeover.

But according to Twitter, many of these former workers still have access to Twitter’s trade secrets and other confidential information. Twitter alleges that Meta took advantage of this and tasked these employees with developing a “copycat” app “in violation of both state and federal law.”

As a result, Twitter is threatening legal action in the form of “both civil remedies and injunctive relief.” It also “demands that Meta take immediate steps to stop using any Twitter trade secrets or other highly confidential information” and says Meta isn’t allowed to crawl or scrape Twitter’s data, either.

Meta responded to Twitter’s letter in a post on Threads, with communications director Andy Stone stating, “No one on the Threads engineering team is a former Twitter employee — that’s just not a thing.” Meta doesn’t seem all too concerned about this, and that may be because Twitter isn’t all that shy about threatening legal action. In May, Twitter accused Microsoft of abusing the company’s API through integrations with some of its products.

Meta launched Threads on Wednesday night, with celebrities and brands the first to get on board. Less than 24 hours since the app’s launch, Threads has garnered over 30 million registered users, while internal data obtained by The Verge’s Alex Heath indicates that users have already made over 95 million threads.

“Competition is fine, cheating is not,” Musk said in a reply to a post about the letter on Twitter. More fuel for the cage match, I guess.

Update July 6th, 4:15PM ET: Updated to add a response from Elon Musk.