Facebook says that it has found no evidence of ‘systematic’ political bias related to its Trending Topics section, but acknowledges the possibility that rogue employees could have impacted the controversial feature.

The tech heavyweight has been thrust into the spotlight following a Gizmodo report that stories about conservative topics were prevented from appearing in Facebook’s trending module.

“Our investigation could not exclude the possibility of isolated improper actions or unintentional bias in the implementation of our guidelines or policies,” wrote Facebook’s General Counsel Colin Stretch, in a letter to Sen. John Thune, chairman of the Senate Committee on Commerce, Science and Transportation, who is investigating the allegations of political manipulation.

Citing an unnamed former journalist that worked on Trending Topics, Gizmodo reported earlier this month that stories about the CPAC meeting, Mitt Romney, Rand Paul, and other conservative topics were prevented from appearing in the trending module. The stories were trending organically among Facebook’s users, the report said.

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Facebook reiterated its recent denial of any bias regarding political content in the letter to Thune.

“Our investigation has revealed no evidence of systematic political bias in the selection or prominence of stories included in the Trending Topics feature,” wrote Stretch. “In fact, our analysis indicated that the rates of approval of conservative and liberal topics are virtually identical in Trending Topics.”

However, in an attempt to improve the service and “minimize risks where human judgment is involved,” Facebook is making a number of changes to Trending Topics, according to the general counsel. The company has already updated its guidelines and conducted refresher training for all reviewers, he explained. It is also implementing “additional controls and oversight” around the review team, as well as “robust escalation procedures.”

Additionally, the social network, which has more than 1.6 billion users, is dropping its reliance on news outlets to help determine what gets posted as a trending topic.

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In the letter to Thune, Facebook said that reviewers accepted topics related to both the 2015 and 2016 CPAC events. “In 2016, although topics related to CPAC were accepted on other days of the conference, one topic related to CPAC itself was not accepted on its first day, March 2, 2016,” he wrote. “Our investigation concluded that this decision was likely the result of the fact that on that day -- the day after the Super Tuesday electoral contests -- reviewers accepted at least 15 topics related to the Republican presidential primary.”

The firestorm over the Gizmodo report prompted Facebook to invite leading conservatives to the company’s headquarters in Menlo Park, Calif., last week to discuss the allegations. Following the meeting, Dana Perino, co-host of Fox News Channel’s “The Five,” said that Facebook recognizes that it has a “trust problem” with conservatives.

Several unnamed former Facebook “news curators” told Gizmodo that they were told to artificially “inject” certain stories into the trending news section, even if they weren’t popular enough to justify inclusion. In some cases the stories weren’t even trending at all, according to the report.

The former curators, who were all contractors, also were reportedly instructed to not add news about Facebook into the trending section. The curators interviewed by Gizmodo worked for the social network across a timespan from mid-2014 to December 2015.

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In the letter, Stretch said that Facebook’s Trending Topics “injection tool,” which is rarely used today, was used more frequently in the past to address limitations in the module. “We found no evidence that the injection tool was used to advance any particular political agenda,” he wrote. “Over time, adjustments to the trending algorithm have dramatically reduced the need for its use.”

The general counsel explained that Facebook uses algorithms to “surface” trending topics and personalize them for each user. However, Stretch explained that human intervention is also needed to overcome current limitations in the algorithm. “To sort the meaningful trends from gibberish and duplicates, and to write headlines and descriptions in clear, natural-sounding language,” he wrote.

The trending section, which appears to the right of the Facebook news feed, was introduced in January 2014. Facebook describes the module as a product “designed to surface interesting and relevant conversations in order to help you discover the best content from all across Facebook.”

Thune, who met with Stretch May 18, described the company’s letter as a step in the right direction. “Facebook has recognized the limitations of efforts to keep information systems fully free from potential bias, which lends credibility to its findings,” he said, in a statement Monday. The company, he added, “has been forthcoming about how it determines trending topics, and steps it will take to minimize the risk of bias from individual human judgment.”

The Associated Press contributed to this report.

Follow James Rogers on Twitter @jamesjrogers