The facts of Sept. 11, 2001, are uncontested: Terrorists hijacked four jetliners, flew two into the World Trade Center and one into the Pentagon, while passengers on the fourth plane helped crash it in Pennsylvania.

After the attacks, conspiracy theories spread quickly. By the middle of George W. Bush’s presidency, a third of the public either believed that the U.S. government assisted in the attacks or took no action to stop them.

Today, 9/11 conspiracy theories remain widespread: 1 in 6 Americans think Bush administration officials knew about the attacks and intentionally let them happen so they could wage war in the Middle East. Others go further, arguing that the government planned and executed the attacks.

These groundless theories — commonly known as “Trutherism” — raise important questions. How does a conspiracy theory take hold? And why, 20 years after the attack, does it endure?

The birth of a conspiracy theory

Trutherism emerged almost immediately after the attacks. By late 2001, some anti-Bush protesters were carrying signs saying “Bush Knew” at rallies. In 2002, Democratic Rep. Cynthia McKinney of Georgia called for an investigation into “What did this administration know, and when did they know it?,” asking “What do they have to hide?”

Mark Fenster, a University of Florida legal scholar, told us that Truthers initially were a mix of everyday conspiracy theorists who tend to believe any number of dark narratives, and liberals who distrusted Bush. Not even the release of the 585-page report of the bipartisan 9/11 Commission, detailing the intelligence failures that led to the attack, slowed the movement.

In a variety of polls in the first decade after 9/11, Democrats were more likely than Republicans to believe in Trutherism. In a 2006 Ohio University survey, 51 percent of Democrats signed on to some form of Trutherism, while only 18 percent of Republicans did the same. The survey is imperfect — it doesn’t include a “somewhat unlikely” option, and other polls show much higher percentages that “don’t know” — but it demonstrates that Trutherism had a solid foothold on the left.

Professors Evan Laine and Raju Parakkal of Thomas Jefferson University studied the psychological drivers of 9/11 Trutherism and why conspiracy theories spread. We explore their findings about trauma in this short film:

Conspiracy theories blaming George W. Bush for the Sept. 11, 2001, terrorist attacks have been debunked, yet millions of Americans still believe them. (Kate Woodsome, David Byler/The Washington Post)

Meanwhile, truther communities cropped up online and believers met at conferences across the country. A new class of “experts” also emerged. Fenster explained that academics such as physicist Steven Jones and theologian David Ray Griffin, as well as longtime conspiracy theorists including Alex Jones, made the case for Trutherism in books, broadcasts and lectures. Within a few years, an entire Trutherism industry had formed.

Polls varied in their exact wording and results, and some voters might have been “expressively responding” — that is, telling pollsters they believed in Trutherism as a way to express deep disapproval with Bush. But by the time Barack Obama took office, at least a quarter of Democrats claimed to believe in Trutherism.

Trump and the changing face of Trutherism

After Bush left office, conspiracy-theory enthusiasts found new obsessions, such as Obama’s birth certificate. But in 2016, amid the rise of Donald Trump (and eight years of Obama as president), Trutherism became more prevalent among Republicans.

By the time Trump took office, the partisan divide on Trutherism had nearly disappeared.

One reason: Once Trump took over the GOP, Bush became a less polarizing figure.

To some on the left, Bush was vastly preferable to Trump. Half of Democrats viewed Bush favorably by January 2018.

Trump spent months pillorying Bush during the 2016 primary campaign, and Bush occasionally made thinly veiled critiques of Trump’s immigration policy and political style. These conflicts left a mark: 1 in 5 Republicans viewed Bush unfavorably by 2018.

Meanwhile, Trump embraced conspiracy theories around election fraud, Obama’s citizenship and other topics. And, although Trump didn’t endorse Sept. 11 Trutherism specifically, he tiptoed around the edges of it and elevated leading Truthers such as Jones.

Conspiracy theories existed in both parties before Trump became the Republican standard-bearer. But Trump brought conspiratorial thinking into the mainstream, encouraging his followers — explicitly or implicitly — to trust no one else and follow him down the rabbit hole.

A new forever conspiracy theory?

Twenty years on, Trutherism remains substantial. One in 6 Americans — including 19 percent of Democrats and 14 percent of Republicans — believe in some form of the conspiracy theory.

To be sure, Republicans are currently more supportive of dangerous conspiracy theories, such as election fraud and fearmongering about coronavirus vaccines. But the data on Trutherism shows there is an audience for conspiracy theories in both parties. A sizable subset of voters are prepared to believe things for which there is no evidence. America will have to cope with that for years to come.