Even in a country now completely inured to the horrors of mass shootings, the massacre at Sandy Hook remains lodged in the minds of everyone old enough to remember it. Ten years ago, 20-year-old Adam Lanza fired 154 rounds from an AR-15-style rifle in less than five minutes. Twenty extremely young children and six adults were killed.

It was the worst elementary school shooting in American history.

Elizabeth Williamson’s new book is about that “American Tragedy”, but more importantly it is about “the Battle for Truth” that followed. In excruciating detail, Williamson describes the unimaginable double tragedy every Sandy Hook parent has had to endure: the murder of their child, followed by years and years of an army of online monsters accusing them of inventing this unimaginable horror.

Alex Jones of Infowars is the best-known villain of this ghastly narrative. His Facebook pages and YouTube channels convinced millions of fools the massacre was either some kind of government plot to encourage a push for gun control or, even more obscenely, that it was all carried out by actors and no one was killed at all.

While a single deranged shooter was responsible for the original tragedy, Williamson makes clear she believes Facebook and Google (the owner of YouTube) deserve most of the blame for the subsequent horror the relatives of victims have endured.

As Congressman Ro Khanna reported in his new book, Dignity in a Digital Age, an internal Facebook document estimated that “64% of all extremist group joins are due to our recommendations”.

Those recommendations are the result of the infernal algorithms which are at the heart of the business models of Facebook and YouTube and are probably more responsible for the breakdown in civil society in the US and the world than anything else invented.

“We thought the internet would give us this accelerated society of science and information,” says Lenny Pozner, whose son Noah was one of the Sandy Hook victims. But “really, we’ve gone back to flat earth”.

It horrified Pozner “to see the image of his son, smiling in his bomber jacket, passed round by an online mob attacking Noah as a fake, a body double, a boy who never lived”. Relentless algorithms pushed those “human lies to the top” on an internet which has become an “all-powerful booster of outrage and denial”.

Noah’s mother, Veronique De la Rosa, was another victim of Jones’s persistent, outrageous lies.

“It’s like you’ve entered the ninth circle of hell,” she tells Williamson. “Never even in your wildest, most fear-fueled fantasies would you have guessed you’d find yourself having to fight not only through your grief, which you know is at times paralyzing, but to even prove that your son existed.”

Lenny Pozner regularly saw his dead son described as an “alleged victim”, but it took years before Facebook and YouTube and Twitter took substantial action to quiet the insane conspiracies their own algorithms had done so much to reinforce.

Facebook allowed a Sandy Hook Hoax group and dozens of others to operate virtually uninterrupted for two years. Hundreds of videos on YouTube, owned by Google, wallowed in the hoax, drawing thousands who made threats against the families.

“Facebook and Twitter are monsters,” says Pozner. “Out-of-control beasts run like mom-and-pop shops.”

Facebook “focuses on growth, to the exclusion of most everything else”, Williamson writes. The algorithms are designed to keep all of us on the platform as long as possible, and they operate “with relentless, sometimes murderous neutrality, rewarding whatever horrible behavior and false or inflammatory content captures and retains users”.

In 2018, an open letter to Mark Zuckerberg by Pozner and De La Rosa, published in the Guardian, accused the Facebook chief of “allowing your platform to continue to be used as an instrument to disseminate hate”.

Two days later, Facebook finally suspended Jones’s personal page but “took no action against Infowars’ account, which had 1.7 million followers”. YouTube removed four videos from Infowars’ channel and banned Jones from live-streaming for all of 90 days.

Harry Farid heads the school of Information at the University of California Berkeley. He describes the perfect storm which is threatening democracy and civility everywhere: “You have bad people and trolls and people trying to make money by taking advantage of horrible things … you have social media websites who are not only welcoming and permissive of it but are promoting it, and then you have us, the unsuspecting public.”

Williamson makes the important and usually forgotten point that nothing in the first amendment gives anyone the right to use a social media platform. All it says on this subject is that “Congress shall make no law … abridging the freedom of speech, or of the press.”

As Farid points out, platforms have an absolute right to ban all forms of content “without running afoul of the constitution” – which is why Donald Trump had no judicial recourse when he was banned from Facebook and Twitter after the Capitol riot.

But most of the time, Farid says, the platforms just look the other way: “Because they’re making so much goddamned money.”

Last fall, the California Democrat Adam Schiff told the Guardian the blockbuster testimony of the Facebook whistleblower Frances Haugen would finally be enough to spur Congress to regulate the worst excesses of the big platforms.

So far, their lobbyists have prevented such action. On Saturday, the Guardian asked Schiff if he still believed Congress might act in this session.

“There is still a chance,” he wrote in an e-mail, adding that “the degree of Russian propaganda on social media attempting to justify their bloody invasion of Ukraine” might finally provide the “impetus” needed for change.

• Sandy Hook: An American Tragedy and the Battle for the Truth is published in the US by Dutton Books