TikTok is under scrutiny after Senators Marsha Blackburn and Richard Blumenthal sent a letter demanding details about a 2021 algorithm experiment that withheld safety features from millions of users.

The experiment, aimed at reducing "filter bubbles" in the For You feed, was tested on a control group while the broader safety feature remained disabled. Chase Nasca, a U.S. teen who died by suicide, was among those in the control group, reportedly exposed to repetitive content about suicide and self-harm.

Internal documents suggest the teen fell into a filter bubble with harmful recommendations leading up to his death. The senators criticized TikTok for prioritizing engagement metrics over user safety, calling the disclosures "sinister."

Lawmakers also questioned whether TikTok has ever withheld safety features due to potential impacts on engagement or ad revenue. A response is due by September 1.

Both senators are co-sponsors of the Kids Online Safety Act (KOSA) and have previously pushed social media platforms for stronger teen protections. TikTok did not immediately respond to requests for comment.