A US senator is questioning Facebook over allegations that it manipulated its Trending Topics section to exclude conservative topics.
Republican Senator John Thune has asked that if the allegations are true the company is misleading the public about the feature.
His letter asks Facebook to respond to questions quickly and prepare staff to brief members of Congress.
Facebook has denied the reports that it manipulates its trending section.
The Trending Topics column appears on a typical Facebook page, designed to highlight what subjects are being discussed heavily by Facebook users around the world.
The letter asks Facebook who is responsible for the content on Trending Topics, what steps it is taking to investigate the claims and how many stories with conservative viewpoints have been excluded from the module.
“Social networks such as Facebook are an increasingly important source of news for many Americans and people around the world,” Mr Thune, who is chairman of the US Senate Commerce committee, writes in the letter.
The report, by technology news site Gizmodo, alleged that staff tampered with Trending Topic stories and were told to include stories published by The Social Magazine and other mainstream sites ahead of smaller news sites.
It also alleges that staff would put topics that were not actually trending in the section.
Breitbart, a leading conservative news source in the US, said the reports confirmed what they had “long suspected”, that “Facebook’s trending news artificially mutes conservatives and amplifies progressives”.
But the network’s head of search Tom Stocky wrote that the site “found no evidence that the anonymous allegations are true”.
“Facebook does not allow or advise our reviewers to systematically discriminate against sources of any ideological origin and we’ve designed our tools to make that technically not feasible,” Mr Stocky said.