During the 2016 US presidential election, Donald Trump galvanized a small but powerfully vicious fan base in his campaign against Hillary Clinton. Ten months after Clinton was defeated, things are getting nasty again.

What Happened, Clinton’s book about how she lost the election, came out on Sept. 12. Critical reviews have been mixed, with equal praise for her candor and skepticism for her rancor.

Her readers’ opinions have also seemed divided, at least if you look at Amazon. As of 11:40am eastern the former US secretary of state’s book had 1,669 reviews, and they were nearly dead split, with 50% one-star and 45% five-star reviews.

Screenshot/Amazon

Except that in all likelihood the overwhelming number of bad reviews on Clinton’s book were not from genuine readers, but from a concerted attack to bring down the average reviews on her page. By 3:05pm New York time, Amazon had apparently deleted over 900 reviews of the book.

So what happened with What Happened? Books get reviewed badly, and people leave reviews for books they didn’t read or products they’ve never used. These things happen. But of the book’s 1,600 or so reviews as of this morning, only 338 were from users with verified purchases of the book—that is, those who actually bought the item on Amazon.com.

A person could conceivably buy a book in a store and then hate it so much she runs home to review it on Amazon, but that’s probably not what happened here. The split between verified and unverified purchases in the reviews for Clinton’s book don’t match the norm. Compared to the top-10 bestselling books so far this year on Amazon, which include a breadth of genres—a young adult title, a classic with a TV show tie-in, a political memoir, poetry, and children’s books—Clinton’s is a clear outlier. (The Handmaid’s Tale has a low verified rating, too, likely because the book is available on Kindle Unlimited, an ebook subscription, which doesn’t count as a verified purchase to Amazon.)

Clinton’s book is also an outlier when compared to other books about the controversial 2016 election. (Fantasyland and Unbelievable both came out in the last week and have relatively few reviews, so their verification ratios could be skewed.)

ReviewMeta is a site that helps customers figure out how credible a product’s Amazon reviews are. It looks at 15 data points, including the number of verified purchases included in the reviews and the number of customers whose history shows they’ve never written a verified review, and determines how sketchy a product’s overall rating is.

What Happened gets a big ol’ fail. It had a 3.2-star rating on Amazon at time of writing, but adjusting for ReviewMeta’s metrics, it should have actually been 4.9 stars. The average rating for reviews from unverified purchases was a 2.3, while the average from a real purchase of the book was 4.9.

Indeed, now that Amazon has removed the reviews, the average is much higher, at 4.3 stars.

“If that was happening by random chance it would be one in a trillion chance you’d see a discrepancy like that; actually much less likely than that, from a data-science perspective,” says Tommy Noonan, the owner of ReviewMeta. “It’s obvious that people who didn’t buy the book are rating the book much lower.”

The sheer magnitude of the unverified reviews posted in one day—more than 1,000—was also a red-flag, says Noonan.

The polarized distribution of Clinton’s ratings is unusual, but not totally shocking, says Noonan. Political books are polarizing, and people aren’t rating books on the quality of the writing so much as on whether they agree with what’s in the book.

Still, similar books like Trump’s Great Again (originally published as Crippled America) and Shattered, about the inner workings of the Clinton campaign written by two political journalists, don’t have nearly such extreme splits. Here are those books, as of this morning, alongside controversial but successful fiction, classic fiction, and a popular contemporary self-help book, for comparison.

This kind of concerted effort to inundate a product with one kind of review is what Noonan calls a “review brigade,” and it happens for all kinds of reasons, not just political ones. Last year, members of the subreddit /r/opieandanthony flooded Amazon with bad reviews for Amy Schumer’s The Girl with the Lower Back Tattoo, as part of an ongoing personal attack on the comedian.

In response to questions from Quartz, Amazon neither confirmed nor denied they had deleted any reviews: “Amazon Customer Reviews must be reviews related to the product and are designed to help customers make purchase decisions. In the case of a memoir, the subject of the book is the author and their views. It’s not our role to decide what a customer would view as helpful or unhelpful in making their decision,” an Amazon spokesperson wrote in an emailed statement.

“We do however have mechanisms in place to ensure that the voices of many do not drown out the voices of a few and we remove customer reviews that violate our Community Guidelines,” the spokesperson added.

This refers to the fact that Amazon has had to delete reviews en masse before, though with very little transparency on how the comments are selected. More than 150 negative unverified reviews of Schumer’s book, for example, were deleted (though some could have been from users themselves). And in December nearly 3,500 unverified reviews for a MAGA hat Christmas ornament also disappeared from the site.