A large number of right-wing and conservative Twitter accounts, including Breitbart News journalists, have been tagged with a mysterious Twitter filter that appears to block their tweets from some searches. It is unclear what else the filter does.

Over the past few days, conservative and right-wing Twitter users have been using the website shadowban.eu to test if the reach of their content is being artificially limited or hidden by Twitter, a process known as “shadowbanning.”

By adding the html tag “&qf=on” and “&qf=off” to the end of search result URLs, the site claims to test whether an account’s tweets are being hidden behind Twitter’s “quality filter,” which hides tweets from what Twitter calls “low quality accounts.” The technology how-to website ghacks also recommends this method to enable or disable the quality filter. “&qf=on” switches on the filter, while “&qf=off” turns it off.

When the turned on, quality-filtered tweets do not appear in search results. When turned off, they do.

Here is an example of the quality filter in action on a tweet from Breitbart News reporter Ian Mason. Mason tweeted about the quality filter shadowban on Friday:

Now compare the search results found on the following two links, which are searches designed to find this exact tweet. The first link has the quality filter enabled, the second does not. Note that the two URLs are exactly the same except for the “qf” setting:

https://twitter.com/search?f=tweets&vertical=default&q=https%3A%2F%2Ft.co%2FDE9iCJAJLq&qf=on

https://twitter.com/search?f=tweets&vertical=default&q=https%3A%2F%2Ft.co%2FDE9iCJAJLq&qf=off

This test demonstrates the quality filter shadowban. Any account with the qualify filter enabled — which is the default setting for all Twitter accounts — would not be able to find that tweet by searching on the site.

Breitbart News reporters Ian Mason, Charlie Nash, John Binder, Jack Montgomery, and Chris Tomlinson, Rebel Media commentator Martina Markota, identitarian activist Martin Sellner, Zionist Organization of America adviser Arthur Schwartz, lawyer and former MAGA meets organizer Will Chamberlain, and many more.

The shadowbanning extends beyond the world of journalists and pro-Trump personalities. Andrew Surabian, spokesman and adviser for Donald Trump Jr., has also reported that his account is shadowbanned.

Curiously, I too was subject to the quality filter shadowban — until people started tweeting about it. As of today, it’s been removed. It’s also possible that the filter targets tweets rather than users — shadowban.eu uses a “reference tweet” to test the quality filter, scraped from a user’s timeline — but that reference tweet frequently changes.

Either way, it doesn’t seem to be affecting left-wing accounts. Tariq Nasheed, a black supremacist and self-described “identity extremist,” is not subject to any filter, according to shadowba.eu. Neither is Louis Farrakhan, the left-wing black supremacist and anti-semite. Nor are hate-filled, obscenity-spewing left-wing celebrities like Peter Henry Fonda.

Aside from censoring certain tweets in search results, it is unclear what the “qf” filter does. Twitter claims its quality filter, which is enabled on all accounts by default, prevents tweets from “low-quality accounts” appearing in your notifications. But when tested, activity from accounts that were tagged with the “qf” filter did appear in notifications, even with the setting enabled.

Perhaps that’s to be expected, though. The point of any kind of under-the-radar account restriction (like a shadowban, which Twitter employees have admitted to using) is that the targeted user should find it difficult to detect. The goal is to have the user continue to tweet as normal, unaware that their account has been limited. The discovery of QF filtering has led to the creation of a petition demanding an end to social media censorship that is gaining traction.

Tech companies are currently facing pressure by national governments to censor the flow of information and prevent stories from alternative news sources from spreading. Earlier this year, Twitter announced that it would censor “behaviors that distort and detract from the public conversation” by downranking content that exhibits such behaviors from search results and “public conversations.” The QF filter, which makes certain tweets impossible to find in search results, seems to do just that.

Allum Bokhari is the senior technology correspondent at Breitbart News. You can follow him on Twitter, Gab.ai and add him on Facebook. Email tips and suggestions to allumbokhari@protonmail.com.