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The chairman of the powerful Senate Commerce Committee asked Facebook CEO Mark Zuckerberg to respond to allegations that the social network's news curators actively suppress conservative viewpoints and articles.

Committee Chairman Sen. John Thune wrote that Facebook has emerged as an important source of news and civil discourse for millions of Americans. If the allegations of routine censorship and politically motivated news manipulation reported by Gizmodo are true, it undermines the social network's claims that it serves as an open, neutral platform.

“Facebook must answer these serious allegations and hold those responsible to account if there has been political bias in the dissemination of trending news,” said Thune on sending the letter. “Any attempt by a neutral and inclusive social media platform to censor or manipulate political discussion is an abuse of trust and inconsistent with the values of an open internet.”

Facebook twice denied the allegations, saying it does not censor the news and trends it shows users on the site's trending section. Rather, that topics are surfaced via an algorithm, and human editors sift through those topics to ensure their relevance and "disregard junk or duplicate topics, hoaxes or subjects with insufficient sources."

Nonetheless, the mere hint of media bias has left Facebook open to blistering criticism from conservatives. Republican National Committee Chairman Reince Priebus quickly seized the opportunity to whip up outrage on another powerful social media platform, Twitter.

Zuckerberg may well have made himself a target by inserting himself into the political arena with his indirect criticisms of the immigration stance of presumptive Republican presidential nominee Donald Trump.

Thune, whose committee oversees tech issues, asked Facebook to provide a full accounting of how the Trending Topics feature works, the role of news curators and the steps the social network has taken in response to allegations of politically motivated manipulation of news stories. He also requested a list of stories removed or "injected" into the trending topics section.

Zuckerberg and Facebook have until May 24 to respond.

Here's the full letter: