Facebook employees are facing questioning from Senate investigators who are probing allegations that the social media company suppresses conservative media outlets in order to boost liberal viewpoints.

“If Facebook presents its trending topics section as the result of a neutral, objective algorithm, but it is in fact subjective and filtered to support or suppress particular political viewpoints, Facebook’s assertion that it maintains a ‘platform for people and perspectives from across the political spectrum’ misleads the public,” Senate Commerce Committee chairman John Thune, R-S.D., wrote in a Tuesday letter to Facebook CEO Mark Zuckerberg.