Twitter threatened to sue Meta just hours after the Instagram parent company launched Threads, an app it hopes will beat out the struggling site owned by Elon Musk.

In a letter to Meta CEO Mark Zuckerberg, published by online news outlet Semafor on Thursday, Musk lawyer Alex Spiro accused the company of “unlawful misappropriation of Twitter’s trade secrets and other intellectual property.”

The letter accused Meta of hiring dozens of former Twitter employees who “had and continue to have access to Twitter’s trade secrets and other highly confidential information.”

Threads is the biggest challenger yet to Musk-owned Twitter, which has seen a series of potential competitors emerge but not yet replace one of the world’s biggest social media platforms, despite its struggles.

Zuckerberg’s latest move against Musk further heightened the rivalry between the two multibillionaires who have even agreed to meet for hand-to-hand combat in a cage match.

And in his first tweet in over a decade, Zuckerberg posted a Spiderman pointing at Spiderman meme in an apparent reference to the similarities between Threads and Twitter.

Threads went live on Apple and Android app stores in 100 countries at 2300 GMT on Wednesday, and early feedback noted its close, but scaled back, resemblance to Twitter.

Within a few hours, more than 30 million people had downloaded Threads, Zuckerberg said Thursday.

“Feels like the beginning of something special, but we’ve got a lot of work ahead to build the app,” Zuckerberg wrote on his official Threads account Thursday.

Accounts were already active for celebrities such as Jennifer Lopez, Shakira and Hugh Jackman, as well as media outlets including The Washington Post and The Economist.

On Threads, Zuckerberg wrote: “It’ll take some time, but I think there should be a public conversations app with 1 billion+ people on it.”

Twitter has said it has more than 200 million daily users.