The U.S. government has launched a national security review of TikTok owner Beijing ByteDance Technology Co’s $1 billion acquisition of U.S. social media app Musical.ly, according to three people familiar with the matter.

While the $1 billion acquisition was completed two years ago, U.S. lawmakers have been calling in recent weeks for a national security probe into TikTok, concerned the Chinese company may be censoring politically sensitive content, and raising questions about how it stores personal data.

TikTok has been growing more popular among U.S. teenagers at a time of growing tensions between Washington and Beijing over trade and technology transfers. About 60% of TikTok’s 26.5 million monthly active users in the United States are between the ages of 16 and 24, the company said this year.