WASHINGTON (Reuters) – Two U.S. senators said on Tuesday a Facebook Inc (NASDAQ:) whistleblower will testify at a Senate hearing next week about what one of them called the social media company’s “toxic effects” on young users.

“This whistleblower’s testimony will be critical to understanding what Facebook knew about its platforms’ toxic effects on young users, when they knew it, and what they did about it,” Democratic Senator Richard Blumenthal, head of a subcommittee of the Senate commerce panel, said in a statement announcing the Oct. 5 hearing.

Senator Marsha Blackburn, the subcommittee’s top Republican, said: “From turning a blind eye to the negative impacts of its platforms on teens’ mental health to its inability to police for trafficking, domestic servitude, and other harmful content, Facebook has a lot to account for.”

Facebook didn’t immediately reply to our request for comment.

It was announced a day after Instagram (the photo-sharing site owned by Facebook) said that the app it is developing for children had been placed on hold amid mounting criticisms from U.S. legislators and advocacy groups.

Although the statement by senators didn’t give much information on the whistleblower, it did include some details.