Two US senators said they were investigating short video sharing app TikTok’s reported decision recently to hire several high-level executives from its Chinese parent company, ByteDance.

Sens. Richard Blumenthal, a Democrat, and Republican Marsha Blackburn said in a letter on Tuesday to TikTok CEO Shou Zi Chew that the personnel moves further call “into question the independence of TikTok’s operations and the security of its US users’ information.”

TikTok, which did not immediately respond to a request for comment, is used by more than 150 million Americans and has faced calls from US lawmakers for a nationwide ban over concerns about possible Chinese government influence.

“The personnel changes give the impression that TikTok is attempting to preserve ByteDance’s influence over TikTok while avoiding suspicion,” the senators wrote, asking for a detailed account of security protocols being imposed on ByteDance employees that transfer from China to the US.

Efforts to give the Biden administration new powers to ban TikTok have stalled in Congress.

Sen. Maria Cantwell has been working with the White House and other lawmakers on a revised bill to address concerns about TikTok and other foreign-owned apps.

Republican Sen. Josh Hawley, who sought unanimously consent to win approval for legislation to ban TikTok in May, plans to force a vote on the issue later this year.

“We need to come back to it and we need to ban it,” he told Reuters last month. “(TikTok) has hired lobbyists by the bazillion, they are in the halls constantly and they have been able to stop progress.”

Then-President Donald Trump in 2020 sought to bar new downloads of TikTok and another Chinese-owned app, WeChat, a unit of Tencent, but a series of court decisions blocked bans from taking effect.

TikTok is fighting a ban by the state of Montana set to take effect on Jan. 1.

A judge has scheduled an Oct. 12 hearing on TikTok’s lawsuit.