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.

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.

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