Forty-seven minutes after news broke of a high school shooting in Parkland, Florida, the posters on the anonymous chat board 8chan had devised a plan to bend the public narrative to their own designs: “Start looking for (Jewish) numerology and crisis actors.”

The voices from this dark corner of the internet quickly coalesced around a plan of attack: Use details gleaned from news reports and other sources to push false information about one of America's deadliest school shootings.

The posters on anonymous forums, a cauldron of far-right extremist politics, over the next few hours speculated about the shooter's ethnicity (“Hope the kid isn't white”) and cracked off-color jokes. They began crafting false explanations about the massacre, including that actors were posing as students, in hopes of blunting what they correctly guessed would be a revived interest in gun control.

The success of this effort would soon illustrate how lies that thrive on raucous online platforms increasingly shape public understanding of major events. As much of the nation mourned, the story concocted on anonymous chat rooms soon burst onto YouTube, Twitter and Facebook, where the theories surged in popularity.

“There's a war going on outside, no man is safe from,” said a frequent conspiracy theory poster on the website Reddit, the day after the shooting. “And it is only partially being fought with guns. The real weapon is information and the attack is on the mind.”

A Washington Post review of thousands of posts on sites such as 8chan, 4chan and Reddit showed how people on online forums worked aggressively to undermine news reports about a troubled teen accused of killing 17 people, mostly students and faculty members.

Former YouTube engineer Guillaume Chaslot called coordinated campaigns across online platforms to make a video trend “4chan attacks” because such anonymous forums often served as staging grounds for these efforts. YouTube and Facebook have policies against harassment that served as the basis for removing some of the conspiracy theories. But Chaslot said the companies have not done enough to weed out deceptive content.

The Parkland storyline took advantage of emerging details about the surviving students – and even how they looked or talked during interviews with television reporters – to portray them as “crisis actors” playing the roles of victims in a “false flag” attack, a hoax designed to mislead the public and build support for gun control.

The campaign threatens to leave long-lasting scars on students and their families. Survivors of the Sandy Hook Elementary School shooting in 2012 and victims' families still are haunted by these online echoes of their trauma, experts say.

“The parents and the families of these murdered children are still being harassed by people online,” said Joan Donovan, a researcher at the think tank Data & Society who studies media manipulation. “For years to come, these students in Parkland are going to have to suffer this same fate.”

Parkland student David Hogg, 17, emerged as the centerpiece of the “crisis actor” myth, in part because his father is a retired FBI agent, allowing the “false flag” idea to merge with ongoing conservative attacks on the bureau related to its investigation of President Donald Trump's campaign.

There was little sign on the chat boards of any unease about singling out Parkland survivors and their families. Instead the mood seemed jubilant, with posters celebrating that the campaign had reached a broader audience of so-called “normies,” meaning people who typically keep their distance from racist, anti-Semitic and far-right extremist conversation.

“Just wanted to say thanks for all your digging and research,” one poster wrote on 8chan. “Extra thanks if you're spreading info or memes about this kid. It's already breaking through the normie-sphere. KEEP PUSHING!”

Anonymous online forums have long incubated politically extreme, racially charged conversation with few rules or concessions to good taste. On 4chan, founded in 2003 and now owned by a Japanese businessman, such chats typically happen on the /pol/ – for “politically incorrect” – message board. 8chan, founded in 2013 by those who considered 4chan too restrictive, also has its own /pol/ board, where the exchanges play out under the heading, “On the jews and Their Lies.”

Reddit is typically regarded as more mainstream, but the individual message boards, including “r/The_Donald” and “r/conspiracy,” hosted harsh attacks on the Parkland students. The site in 2016 closed its thriving “Pizzagate” conspiracy theory message board, a leading source of allegations that a child molestation ring run by Democratic Party luminaries operated out of a Washington, D.C., pizza shop that led to a real shooting in which no one was hurt.

As coverage of the Feb. 14 shooting spread rapidly, the posters of 8chan's /pol/ board were among the first to begin assembling scraps of what they claimed was proof of a hoax. They criticized how the students acted, mocked how they looked and shared crude memes and photos taken from the students' social-media accounts.

By the end of that first day, the conspiracy theorist Alex Jones raised the possibility on his Infowars show that the shooting was a “false flag” attack. In a later interview, Jones said he soon became convinced that the attack was real, even though he continued to raise questions about other aspects of its portrayal on mainstream news reports.

But the “false flag” idea rapidly was gaining popularity. Within hours, “crisis actor” theories could be found in a network of YouTube videos, tweets and forum threads picking apart public information about the students.

The political stakes were explicit. One poster on 4chan said in all-caps fury, “HILLARY IS TRYING TO TAKE OUR GUNS AWAY XD!!!!!!!!!!!” (“XD” is commonly used on message boards to suggest laughter.)

On 8chan, posters fretted over the potential political consequences of images that the suspect, Nikolas Cruz, posted to his Instagram page including guns, a box of ammunition and a picture of himself wearing a red “Make America Great Again” hat. One poster said, “another 'gun nut' narrative incoming.” Another said, “that maga hat is going to get trump lynched in the media if it turns out to be this kid.”

In interviews, Hogg mentioned that his father had worked for the FBI, a revelation treated by the far-right online ecosystem as a smoking gun worthy of widespread sharing in tweets and memes. A woman that same day posted a video analyzing the students' body language and fiercely criticized Hogg's comments in one interview as those of a “psychopath.”

When users succeeded in falsely convincing some researchers and journalists that the shooter was an active member of a white-supremacist group, using a tactic Donovan calls “source hacking,” one anonymous 8chan user posted, “We will double down on this being another attack against white interests,” urging that images of Cruz be altered before posting.

“Make them darker in photoshop, and make his lips and nose thicker and wider a small bit,” the poster wrote. “Enough to where he looks even more non-white, but not enough that it's too noticeable.”

By Feb. 20, some online posters said they'd clinched a win when Benjamin Kelly, an aide to a Florida state representative, touted conspiracy theories about the shooting, leading to the aide's firing. But the biggest surge of popularity came a day later, when YouTube's No. 1 “Trending” video labeled Hogg an “actor.”

There also was a backlash brewing. YouTube blocked videos pushing allegations about the Parkland students being “crisis actors” as violating its harassment policy. Jones said several of his Infowars videos were among those blocked.

But Jones was unrepentant in alleging that the students – even if they were authentic survivors of a mass shooting – were being coached as part of a politically motivated gun-control campaign.

“Parents who put their kids out on TV knew what that comes with,” he said. “Free speech is a rough thing.”