Spiro, in his letter to Mark Zuckerberg, accused Meta of hiring former Twitter employees who "had and continue to have access to Twitter's trade secrets and other highly confidential information," News website Semafor first reported.
"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," Spiro wrote in the letter.
A Reuters source with knowledge of the letter confirmed its contents on Thursday. Spiro did not respond to a Reuters request for comment.
"No one on the Threads engineering team is a former Twitter employee — that's just not a thing," Meta spokesperson Andy Stone said in a Threads post.
Meta, which launched Threads on Wednesday and has logged more than 30 million sign-ups, looks to take on Elon Musk's Twitter by leveraging Instagram's billions of users.
Meta owns Instagram as well as Facebook.
To press a trade secret theft claim against Meta, Twitter would need much more detail than what is in the letter, said intellectual property law experts including Stanford law professor Mark Lemley.
"The mere hiring of former Twitter employees (who Twitter itself laid off or drove away) and the fact that Facebook created a somewhat similar site is unlikely to support a trade secrets claim," he said.
Jeanne Fromer, a professor at New York University, said companies alleging trade secret theft must show they made reasonable efforts to protect their corporate secrets. Cases often revolve around secure systems that were circumvented in some way.
The newest challenge to Twitter follows a series of chaotic decisions that have alienated both users and advertisers, including Musk's latest move to limit the number of tweets users can read per day.
Since Musk's takeover of the social media platform last October, Twitter has received competition from Mastodon and Bluesky among others. Threads' user interface, however, resembles the microblogging platform.
A former senior Twitter employee told Reuters they were not aware of any former staffers working on Threads, nor any senior personnel who landed at Meta at all.
Meanwhile, Twitter owner Musk said, "Competition is fine, cheating is not," in response to a tweet citing the news.