A report published in the Guardian today includes a training document for Facebook editors that outlines how they should curate the "trending" headlines tool.

Instructions in the documents appear to contradict Facebook's earlier denials of a Gizmodo article that said Facebook editors directly inserted headlines into the trending news widget. It also contradicts what Facebook told Recode last summer.

The guidelines obtained by the Guardian additionally show that editors must select trending news topics from headlines its algorithm surfaced, and make sure that they appear on five of a list of 10 "trusted" news sources that include the New York Times, BBC News, CNN and Fox News. This is likely to make people who are upset about alleged anti-conservative bias that Gizmodo reported on even madder; Fox News looks like the only right-wing outlet on the list.

The Guardian notes that the former Facebook editors it spoke to disagreed with the allegations of anti-conservative leanings at Facebook, and that Facebook has since updated its guidelines.

Shortly after this story was published, Facebook VP of global operations Justin Osofsky posted a broad overview of how the Trending Topics tool works, in addition to Facebook's updated Trending Topics guidelines.

These guidelines include a list of the many RSS links for news publications that Facebook scans to find news topics. Additionally, Facebook released the list of the 1,000 publications that it uses to verify a generated trending topic. Some publications that aren't on the so-called "1K" list: Capital New York, Times of India, Eater, Polygon and Recode.

Keeping track of these lists is getting confusing, so here's a straightforward breakdown of how Facebook uses them to source and write its Trending Topics headlines:

The RSS feed is a way of identifying stuff that's trending that isn't just on Facebook.

The 1K list has websites that can be used to verify headlines.

Editors use the 10 outlets list (New York Times, Fox News, CNN, etc.) to figure out how important the story is.

We've embedded both the 1K list and the RSS list at the bottom of this post.

A representative for Facebook directed Recode to a specific line in the leaked guideline, in the "Injecting Topics" section: "The editorial team CANNOT inject a newsworthy topic if it is not appearing in the [news topic] demo or re-view tools."

In a separate statement provided to Recode, which paraphrases the blog post and is available to read below, Osofsky says that the leaked documents are proof of how seriously Facebook takes its ideological even-handedness:

The guidelines demonstrate that we have a series of checks and balances in place to help surface the most important popular stories, regardless of where they fall on the ideological spectrum. Facebook does not allow or advise our reviewers to systematically discriminate against sources of any political origin, period. What these guidelines show is that we’ve approached this responsibly and with the goal of creating a high-quality product — in the hopes of delivering a meaningful experience for the people who use our service. Trending Topics uses a variety of mechanisms to help surface events and topics that are happening in the real world. In our guidelines, we rely on more than a thousand sources of news — from around the world, and of all sizes and viewpoints — to help verify and characterize world events and what people are talking about. The intent of verifying against news outlets is to surface topics that are meaningful to people and newsworthy. We have at no time sought to weight any one view point over another, and in fact our guidelines are designed with the intent to make sure we do not do so.

Here's the RSS list:

RSS List - FB Trending Topics by Noah Kulwin

And here's the 1K list:

Media 1K List - FB Trending Topics by Noah Kulwin

This story has been updated continually as news has developed.