Facebook has denied allegations that the team responsible for its trending topics section deliberately suppressed conservative views – but says it will improve the feature.

Allegations have been made anonymously that the team responsible for choosing trending topics did so with little oversight and deliberately suppressed conservative views.

On Monday, Facebook denied any bias in a press release and a letter sent directly to John Thune, the chairman of the US Senate commerce committee.

Colin Stretch, general counsel at Facebook, said the company met with Thune on 18 May to discuss an internal two-week investigation, which had found “virtually identical” rates of approval of conservative and liberal topics.

“Suppressing political content or preventing people from seeing what matters most to them is directly contrary to our mission and our business objectives, and the allegations troubled us deeply.”

But Stretch said the investigation “could not fully exclude the possibility of isolated improper actions or unintentional bias” and Facebook would make changes to trending topics to prevent potential misuse and “to minimise risks where human judgment is involved”.

The allegations, first reported by Gizmodo on 9 May, were made by a former “news curator” of the trending topics team. Shortly afterwards, Thune demanded answers from Facebook chief executive Mark Zuckerberg.

Trending topics appear in the upper-right corner of the Facebook website, separate to the main news feed, and are said to be chosen to “help people discover current content that is both popular in the world and meaningful to them”.

Topics are selected through a combination of algorithms and staff. In the letter to Thune, Facebook said: “We currently use people to bridge the gap between what an algorithm can do today and what we hope it will be able to do in the future.”

The allegations of anti-conservative bias prompted Zuckerberg to hold a meeting with conservative commentators last Wednesday.

The platform’s internal investigation spoke to current and former reviewers, current supervisors and “leading conservatives, to gain valuable feedback and insights”.

Stretch said it found “no evidence of systematic political bias” in either the selection or prominence of stories included in the feature. “We were also unable to substantiate allegations of politically-motivated suppression of particular subjects or sources.”

The letter to Thune went into more detail: topics relating to Republican political figures Ted Cruz, Rand Paul, Mitt Romney and Scott Walker – allegedly suppressed by reviewers because of political bias – were in fact accepted “on dozens of occasions”.

“In fact, the two most frequently accepted topics since early 2015 are presumptive Republican presidential nominee Donald Trump and #GOPDebate,” the letter went on.

Analysis of coverage of the late Navy Seal Chris Kyle, the Conservative political action conference, former IRS official Lois Lerner, radio host Glenn Beck, the Drudge Report, and comedian Steven Crowder in trending topics also did not support claims that conservative news stories were suppressed, Facebook said.

Nonetheless, Facebook said its guidelines have been clarified and reviewers have undergone refresher training “that emphasised content decisions may not be made on the basis of politics or ideology”.

Additional controls and oversight around the review team would also be introduced.

Stretch said Facebook would also stop using external websites and news outlets to assess the importance of a story, and would no longer rely on a top 10 list of news outlets as revealed in internal guidelines leaked to the Guardian.

Earlier this month, Facebook released an updated version that showed BuzzFeed News had replaced Yahoo News among the 10 most trustworthy news sites.

Thune said in a statement that he welcomed Facebook’s efforts to address allegations of bias and his concern about “a lack of transparency”.

“The seriousness with which Facebook has treated these allegations and its desire to serve as an open platform for all viewpoints is evident and encouraging and I look forward to the company’s actions meeting its public rhetoric.”