The violent clashes between a pro-Trump demonstration and counterprotesters on April 15 turned Berkeley, Calif., into a battleground of activists on the right and left. On a day when dozens were arrested and injured — one man with a weaponized U-lock stood out.

Many threw punches. He drew blood.

One attack was captured in a profanity-filled video later posted to YouTube. The bike lock flashes through the air and smashes into a man’s head with a thunk.

“Oh s—, he’s bleeding,” one man said on the video, as the victim staggers back and someone shouts for a paramedic. “He’s bleeding! Hard!”

Berkeley police say the man hit at least seven people in the head that day, seriously injuring three.

As the police began their search for a criminal, 4chan’s anonymous message board posters began a search of their own. For them, the man with the bike lock wasn’t just the perpetrator of a violent attack. He was a useful symbol for an increasing focus of 4chan’s hatred: the antifascists, or “antifa,” activists.

Several days after the attacks in Berkeley, 4chan users claimed that the assailant was Diablo Valley College professor Eric Clanton.

Last week, police arrested Clanton, 28, and charged him with four counts of assault with a deadly weapon and one count of wearing a mask.

Clanton’s attorney, Dan Siegel, told The Washington Post his client has pleaded not guilty to the charges.

He emphasized that his client is innocent until proven guilty, but his “life has been upended” by anonymous people on the Internet — particularly people on 4chan.

The users of the message board known for a celebrity nude scandal, pranks and the creation and corruption of the Pepe the Frog meme are also known for their enthusiasm for identifying people they think have done wrong.

Lately, a popular target has been left-leaning activists who show up to pro-Trump rallies and other demonstrations, such as the uproar in Berkeley over the cancellation of conservative Ann Coulter’s scheduled appearance. BuzzFeed reported that in recent months “4chan users have become more and more interested in the identification and doxing of anti-fascist activists.” Doxxing is the making of private or personally identifying information public without the consent of the target.

In the BuzzFeed article, an image of a 4chan discussion thread showed users organizing. “An anti-facism petition went around, which dumb lefties signed, and now we are digging up any info we can find on the people who signed it. … We can make a fun game since the list is so long.”

Online efforts to identify or unmask people or groups have had a troubled history — with real world consequences. A particularly notable case was after the Boston Marathon bombing in 2013, when online sleuths identified the wrong culprits. In 2015, unsuspecting people were inundated with calls accusing them of being associated with the Ku Klux Klan after their names wrongly ended up on an Anonymous-linked list of suspected Klan members.

More recently, a number of people peddled the false conspiracy theory known as Pizzagate, which linked Hillary Clinton to an alleged child-sex-trafficking ring in a D.C. pizza parlor.

Some deliberately use misinformation to paint groups in a negative light, such as untrue claims made over Memorial Day weekend that antifa activists vandalized military cemeteries.

These campaigns originating from 4chan are often motivated by a mix of revenge, boredom and the pursuit of fun.

Recently, users of the message board repeatedly tracked down and disrupted what many took to be an anti-Trump art project involving actor Shia LaBeouf. What may have looked politically-motivated could also have been, as 4chan user Eugene Li said, “the world’s largest capture the flag game.”

Whitney Phillips, a professor of literary studies at Mercer University who has studied 4chan, previously told The Post’s Caitlin Dewey about the dangers of these crowdsourced pursuits.

“You can get information wrong. You can harm people who have nothing to do with it. Your actions can have further repercussions than you expect.”

In short: “Anything that relies on the mob mentality is a powder keg.”

4chan regularly deletes old threads, but archived threads from the site’s /pol/ board show users dissected everything that could be discerned from videos and photos posted online of the incident: the dark sunglasses the lock-wielding attacker wore, the book bag he carried on his shoulders, even his eyebrows. They shared other videos that purportedly show the same man swinging the lock at other people, including one that slowed down the punishing strikes.

One user hit pay dirt: a now-deleted video of a scuffle where the man with the bike lock lost his bandanna. His face was exposed.

It wasn’t long before they claimed to connect the face to a name: Clanton. 

They said they found his Facebook and his Twitter accounts, and dug up what they say was his OkCupid profile. Their unconfirmed findings were eventually picked up by prominent figures on the right, spreading the information to a much larger audience, particularly on Twitter and on r/The_Donald, a popular subreddit for Trump supporters.

Some of the tweets about Clanton in the days after 4chan’s purported identification have since been deleted. One still-live tweet reads, “Nice house, . Would be a shame if something happened to it.” The Twitter user attached an image of what is implied to be Clanton’s home. Multiple archived threads from 4chan and 8chan’s /pol/ boards in the days after the Berkeley protests contain an alleged address for Clanton.

An imgur composite circulated among 4chan users displays nearly two dozen pictures of him and includes details from what they claim was his professor page at the community college.

Clanton, who is out on bail, is scheduled to appear in court this month to set dates for the preliminary hearing. His job duties have been suspended, and he may be fired, said Siegel, his attorney.

“He has to basically hide out from these right-wingers who are … doxxing him, who are publishing what would otherwise be private information,” Siegel said. “People are talking about killing him, about raping him in prison.”

A Berkeley Police Department spokesman would not detail how investigators closed in on Clanton. The spokesman said only that officers “got a lot of tips from the public.” 

But in a news release about Clanton’s arrest, the police said, “For those of you who have already shared photographs and video from the event, we are grateful for your assistance.”

And on 4chan on the afternoon of May 26, before Clanton’s first court appearance, users took credit and celebrated.

“The bike lock bandit’s arraignment is in 40 minutes!” one wrote. “Whos going?”

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