Congressional Republicans are taking their concerns over Facebook’s reported media bias straight to the source. Sen. John Thune (R-SD) wrote a letter Tuesday to Facebook CEO Mark Zuckerberg on behalf of the Senate Commerce, Science, and Transportation Committee requesting additional information on the company’s process for selecting trending news topics.

“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,” Thune wrote, citing the company’s purported neutrality policy.

The letter comes a day after a Gizmodo report asserted that trending news topics on the social network were a reflection of workers’ own political leanings rather than the sole result of a computer algorithm. The report, which quoted a former Facebook news curator, said certain stories were “injected” into the trending news timeline even if they weren’t genuinely popular among Facebook users. The anonymous curator, who Gizmodo identified as politically conservative, said those “injected” stories skewed liberal and that other curators “suppressed” conservative news content. Other anonymous curators interviewed by Gizmodo and Facebook have denied the claims.

Tom Stocky, who heads Facebook’s trending topics team, contested the accusations Monday in a post.

There are rigorous guidelines in place for the review team to ensure consistency and neutrality. These guidelines do not permit the suppression of political perspectives. Nor do they permit the prioritization of one viewpoint over another or one news outlet over another. These guidelines do not prohibit any news outlet from appearing in Trending Topics.

Facebook has previously been accused of not listing social media competitors — namely Twitter — by name when topics are trending on the site. Twitter executives claimed Facebook’s curators referred to the microblogging site generically as “social media.”

But claims that the social network’s news content may not be subjective may force Facebook and other social media companies to more decisively determine how their platforms should be used. Congress’ intervention on the issue reflects Facebook’s influence in news dissemination.

In his letter Tuesday, Thune asked Zuckerberg to investigate the allegations and provide the committee with details on how the company determines what events are chosen to trend, including a record of curators’ decisions and a list of all news stories that may have been injected into the trending topics since 2014.

The company isn’t legally obligated to comply to the letter. However, Thune could be banking on the multibillion-dollar company’s desire to seem welcoming to users across the political spectrum.

Zuckerberg has until May 24 to respond, according to the letter.

John Thune Letter to Facebook on Media Bias by ThinkProgress