Executives from the Chinese-owned video app TikTok have declined to testify at a congressional hearing set for Tuesday that aims to explore the tech industry and its ties to China, a move that threatens to worsen the social-media company’s woes in Washington at a moment when it’s under investigation.

Republican Sen. Josh Hawley (Mo.) organized the session to explore privacy and security concerns posed by Beijing and that government’s strict rules that censor what users can say online and what happens to their data. The decision by TikTok not to attend as of Sunday night was confirmed by a source familiar with the matter who was not authorized to discuss it on the record. TikTok did not respond to a request for comment. A Hawley spokesperson declined to comment.

Congressional lawmakers have grown increasingly concerned about TikTok’s handling of controversial political content and its approach to privacy. Their fears stem from the fact that TikTok is owned by China-based company ByteDance that must censor its services in that country in conformance with the government’s strict censorship demands. ByteDance did not immediately respond to a request for comment.

In recent weeks, TikTok has sought to emphasize its independence, stressing it has never removed a video at the request of Chinese government officials while noting that U.S. users’ data is stored in the United States. It also has said it is not subject to Beijing’s surveillance laws.

But the company’s comments haven’t satisfied some U.S. watchdogs, including the Committee on Foreign Investment in the United States, an interagency group that investigates foreign acquisitions of U.S. companies for national security concerns. CFIUS has opened a probe of TikTok based on ByteDance’s 2017 purchase of the U.S. company Musical.ly, which became TikTok. Republican. Sen Marco Rubio (Fla.) asked for such a probe.

Two other lawmakers – Senate Democratic Leader Chuck Schumer (N.Y.) and Republican Sen. Tom Cotton (Ark.) – have asked U.S. intelligence officials to begin another review of TikTok to determine whether the app poses a national security concern.

TikTok has become one of the fastest growing social media platforms in the world, downloaded more than 1.3 billion times around the world, 120 million of those in the United States.