Facebook announced this week it was sending employees out for retraining and would discontinue some of its practices as it sought to defend itself against charges of political bias against conservatives.

The online giant denied that it’s shown “systematic political bias,” but admitted employees played a bigger role than previously acknowledged in determining what news is highlighted in the trending topics section.

Facebook also acknowledged that it couldn’t rule out the possibility that rogue employees unintentionally discriminated against conservative stories or even acted with malice in “isolated improper actions.”

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In one instance Facebook rejected a story this year about the opening of the annual Conservative Political Action Conference — the largest gathering of right-wing activists in the country. Facebook says that was likely because there were already enough stories about the Republican presidential primary. But the company said since it allowed CPAC posts in 2015, and covered other parts of the 2016 conference, there wasn’t any discrimination.

“Our investigation has revealed no evidence of systematic political bias in the selection or prominence of stories included in the Trending Topics feature. In fact, our analysis indicated that the rates of approval of conservative and liberal topics are virtually identical in Trending Topics,” Colin Stretch, the company’s general counsel, said in an extensive reply to Sen. John Thune, chairman of the Senate Commerce Committee, who is probing the allegations of bias.

Former Facebook employees told Gizmodo earlier this month that they detected bias in the way news was “curated” by the site, with stories on top GOP figures, conservative commentators and right-wing causes getting short shrift.

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Conservative groups chimed in, saying they’d also seen evidence that stories about their issues and actions weren’t getting the attention they’d thought warranted. The American Conservative Union, which organizes CPAC, said stories about the conference on Facebook did poorly, even though they did well on other online platforms such as Twitter — suggesting bias.

Facebook said it probed seven separate allegations, including treatment of CPAC, conservative host Glenn Beck, former IRS senior executive Lois G. Lerner, the Drudge Report and others, and concluded there were no substantiated instances of political bias.

Instead, some stories were rejected because the news sources were questionable, or because the topic was already represented, the company said.

It said liberal and conservative topics saw “virtually identical” treatment, while “moderate” topics fared the best because they were “popular across the political spectrum.”

Still, Facebook said it will retrain employees and impose new “controls and oversight” to try to cut down the chances for bias. It is also discontinuing the use of 10 outlets it used to judge stories’ importance — a list that included Fox News, but also included the New York Times, Washington Post, CNN, NBC News and BuzzFeed.

Mr. Thune said the review was a good start, and said the fact that the company admitted its limitations — including the larger role employees played — “lends credibility” to the findings.

“Facebook’s description of the methodology it uses for determining the trending content it highlights for users is far different from and more detailed than what it offered prior to our questions,” the South Dakota Republican said. “We now know the system relied on human judgment, and not just an automated process, more than previously acknowledged.”

Democrats had blasted Mr. Thune’s questions to Facebook, saying he was wasting taxpayer money in pursuing the probe.

But with nearly two-thirds of Facebook users saying they get at least some of their news from the online platform, conservatives said any chance of bias could skew political conversations.

Mr. Thune said he’s open to new information, but signaled the letter answered most of his questions.

Earlier in the day, though, the ACU sent a letter of its own, asking Mr. Thune to seek out and interview whistleblowers to get to the bottom of their accusations.

“This issue it still unresolved, even after these admissions of wrongdoing by Facebook,” said ACU chairman Matt Schlapp. “Facebook has admitted to harming CPAC, but they have not called us to apologize, and they have failed to explain what they did.”

Facebook CEO Mark Zuckerberg arranged a meeting with conservative figures last week to talk out the issue — a meeting Mr. Schlapp sat out — and tried to assuage concerns. The company’s letter Monday to Mr. Thune also gave more detail about its operations.

Facebook said most users get their news from their news feeds. But the company also runs a “Trending Topics” section that promotes some stories, and that’s where the bias charges focused.

The section was launched in 2014, and was based on an algorithm designed to pick out popular stories.

But the company acknowledged in its reply Monday that employees play a significant role.

“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 — to sort the meaningful trends from gibberish and duplicates, and to write headlines and descriptions in clear, natural-sounding language,” Mr. Stretch said.

He said up to half of the topics selected by the algorithm are rejected by employees because they are duplicates or “do not make sense at the time.”

Some topics that are removed were put on a “blacklist,” meaning they couldn’t be renewed for up to 24 hours, while employees tried to figure out the sourcing of the story or whether the story was already old news.

Facebook said some stories were sidelined because employees questioned the reliability of sources, but said that put too much power in the hands of the reviewers. The company said it will cancel that power as part of its new steps.