Twitter is threatening Meta with a lawsuit after the blockbuster launch of Meta's new Twitter rival, Threads — in perhaps the clearest sign yet that Twitter views the app as a competitive threat.

On Wednesday, an attorney representing Twitter sent Meta CEO Mark Zuckerberg a letter that accused the company of trade secret theft through the hiring of former Twitter employees.

The letter was first reported by Semafor. A person familiar with the matter confirmed the letter's authenticity to CNN.

The letter by Alex Spiro, an outside lawyer for Twitter owner Elon Musk, alleged that Meta had engaged in "systematic, willful, and unlawful misappropriation of Twitter's trade secrets and other intellectual property."

In response to reports on the letter, Musk tweeted: "Competition is fine, cheating is not."

The letter goes on to say that Meta hired former Twitter employees who "have improperly retained Twitter documents and electronic devices" and that Meta "deliberately" involved these employees in developing Threads.

"Twitter intends to strictly enforce its intellectual property rights," Spiro continued, "and demands that Meta take immediate steps to stop using any Twitter trade secrets or other highly confidential information."

Meta spokesperson Andy Stone flatly dismissed the letter. "No one on the Threads engineering team is a former Twitter employee — that's just not a thing," he said on Threads.

In the months since Musk acquired Twitter for $44 billion, the social network has been challenged by a growing number of smaller microblogging platforms, such as the decentralized social network Mastodon and Bluesky, an alternative backed by former Twitter CEO Jack Dorsey. But Twitter has not threatened either with litigation.

Unlike some Twitter rivals, Threads has experienced rapid growth, with Zuckerberg reporting 30 million user sign-ups in the app's first day. As of Thursday afternoon, Threads was the number-one free app on the iOS App Store.

The legal threat may not necessarily lead to litigation but it could be part of a strategy to slow down Meta, said Carl Tobias, a law professor at the University of Richmond.

"Sometimes lawyers, they threaten but don't follow through. Or they see how far they can go. That may be the case, but I don't know that for sure," Tobias told CNN. He added: "There may be some value to tying it up in litigation and complicating life for Meta."