Twitter is wasting no time firing back at Meta, threatening to sue the social media giant over its Threads app, labeling it a “copycat.”

First reported by Semafor, Twitter attorney Alex Spiro sent a letter Meta CEO Mark Zuckerberg, taking issue with Meta poaching Twitter employees that “have access to Twitter’s trade secrets.” The letter accuses Meta of “systematic, willful, and unlawful misappropriation of Twitter’s trade secrets and other intellectual property.”

The letter goes on to threaten legal action:

Twitter intends to strictly enforce its intellectual property rights, and demands that Meta take immediate steps to stop using any Twitter trade secrets or other highly confidential information. Twitter reserves all rights, including, but not limited to, the right to seek both civil remedies and injunctive relief without further notice to prevent any further retention, disclosure, or use of its intellectual property by Meta.

Meta is denying Twitter’s claims, saying there is no trade secret theft.

“No one on the Threads engineering team is a former Twitter employee — that’s just not a thing,” Andy Stone, Meta’s communications director, told Semafor.

There was no doubt things were going to get interesting with Meta’s new app, but no one knew just how interesting until now.