Senate Republicans on Tuesday demanded an explanation from Facebook after reports that former company staffers excluded links to conservative political stories.
A senior Facebook executive said the company has found no evidence to back up the anonymous allegations.
In a letter to chairman and chief executive Mark Zuckerberg, South Dakota Sen. John Thune requested information on who at the company made the decisions on stories for Facebook's Trending Topics feature, what training is provided to employees, whether the company is investigating and what steps it will take to hold people accountable.
"If Facebook presents its Trending Topics section as a 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," wrote Thune, who chairs the Senate Commerce, Science and Transportation Committee.
He asked for answers by May 24.
News reports say Facebook employees excluded links to stories they considered less reliable in its list of trending stories, though individuals could post links to conservative stories on their own Facebook feeds.
Tom Stocky, Facebook vice president of search, said his team is responsible for the Trending Topics and the company has "found no evidence that the anonymous allegations are true."
"There are rigorous guidelines in place for the review team to ensure consistency and neutrality," Stocky wrote in a post. "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."
Adam Jentleson, an aide to Senate Minority Leader Harry Reid, D-Nev., maintained there are more pressing priorities than the Facebook issue.
"The Republican Senate refuses to hold hearings on (Supreme Court nominee) Judge (Merrick) Garland, refuses to fund the president's request for Zika aid and takes the most days off of any Senate since 1956, but thinks Facebook hearings are a matter of urgent national interest," Jentleson said. "The taxpayers who pay Republican senators' salaries probably want their money back."
Questioned later in the day about his inquiry, Thune defended the committee's actions and insisted he wasn't suggesting "anything untoward" on the part of Facebook. He said the panel was responding to media reports and asking for a clarification on policy.
"We think this is perfectly legitimate line of inquiry," he said.
Associated Press writers Matthew Daly in Washington and David Hamilton in San Francisco contributed to this report.