Facebook is being accused of fiddling with its formulas to suppress conservative news.

That’s what some unnamed former Facebook contractors told the tech site Gizmodo—and it’s an accusation that strikes at the heart of the social network’s credibility.

Facebook relies on computer algorithms to determine what is “trending,” an influential designation that inevitably boosts traffic for what are deemed the hottest topics. But unbeknownst to much of the public, Facebook hires journalists to tweak these formulas, and this is where the question of political bias has erupted.

Gizmodo reports that Facebook “routinely suppressed news stories of interest to conservative readers,” according to a former journalist who worked on the trending designations. And several former Facebook “news curators” told the website that they were told to “inject” certain topics into the trending list, even if they weren’t popular enough to warrant making the crucial list.

Depending on who was on duty, said the unnamed conservative ex-curator, citing fear of retribution from the company, “things would be blacklisted or trending … I’d come on shift and I’d discover that CPAC or Mitt Romney or Glenn Beck or popular conservative topics wouldn’t be trending because either the curator didn’t recognize the news topic or it was like they had a bias against Ted Cruz.”