WASHINGTON — The chairman of the Senate Commerce Committee demanded on Tuesday that Facebook explain how it handles news articles in its “trending” list, responding to a report that staff members had intentionally suppressed articles from conservative sources.

In a letter, the chairman, Senator John Thune, Republican of South Dakota, asked Facebook to describe the steps it was taking to investigate the claims and to provide any records about articles that its news curators had excluded or added. Mr. Thune also asked directly whether the curators had “in fact manipulated the content,” something Facebook denied in a statement on Monday.

“If there’s any level of subjectivity associated with it, or if, as reports have suggested that there might have been, an attempt to suppress conservative stories or keep them from trending and get other stories out there, I think it’s important for people to know that,” Mr. Thune told reporters on Tuesday. “That’s just a matter of transparency and honesty, and there shouldn’t be any attempt to mislead the American public.”

Mr. Thune’s actions raised further questions about the content seen by the 1.6 billion people who regularly use Facebook. The platform’s growing role as an arena for news distribution has raised some concerns that it could have undue influence over the flow of information, but any effort by the federal government to regulate or investigate editorial decisions could run into First Amendment protections.