Facebook has admitted to censoring a video posted by activists protesting the Dakota Access pipeline, with the social network blaming the removal on its automated spam filter. The live stream video, published on Tuesday by the media collective Unicorn Riot, showed police arresting around two dozen protesters at a Dakota pipeline site. Unicorn Riot published a link to the live stream on its Facebook page, but the URL was blocked and other users were unable to share it. The link has since been restored, and a Facebook spokesperson apologized for the removal in a statement to Motherboard.

In a statement to Antimedia, a member of Unicorn Riot said that the video was censored "shortly before two of our journalists were arrested onsite," adding that posts and comments that contained the URL "triggered popup security alerts." According to the collective, Facebook's debugger said that the link violated the site's "community standards."

The $3.7 billion Dakota Access pipeline has become a controversial project in North Dakota, sparking protests from the Standing Rock Sioux tribe and other activists who say it would pose health hazards and destroy cultural artifacts. Last week, the US government temporarily halted construction of the 1,170-mile pipeline, which would have crossed an area near the Standing Rock Sioux reservation.

"We're very sorry about this mistake."

In a statement to Motherboard, a Facebook spokesperson said that Unicorn Riot's video was mistakenly flagged by the company's automated spam filters. "The link was removed in error and restored as soon as we were able to investigate," the spokesperson said. "Our team processes millions of reports each week, and we sometimes get things wrong. We're very sorry about this mistake."

Facebook has come under increased criticism recently for the way it polices content on its platform. In July, the company briefly took down a video that showed the fatal shooting of 32-year-old Philando Castile, blaming the removal on a "technical glitch." Facebook used similar language later that month, after it admitted to blocking links to hacked Democratic National Committee emails that were released by WikiLeaks. The company faced widespread criticism earlier this month, when it removed an iconic photograph that showed a naked nine-year-old girl fleeing a napalm bombing during the Vietnam War.

Facebook and other web companies have begun incorporating more automated technologies to control the spread of terrorist propaganda and other extremist material, but civil liberties groups have raised concerns that the companies' algorithmic approach to moderation could infringe on free speech. Facebook has also been mired in an ongoing controversy over its news-gathering algorithms, which have recently promoted inaccurate hoaxes and conspiracy theories.

Speaking to university students in Rome last month, Facebook CEO Mark Zuckerberg reiterated the company's stance that it is not a media company, and that it has no plans to become one. Many would argue that it already is.