I remember precisely where I was and what I was doing when I heard: I was about three weeks into my first year at Emory University in Atlanta, and I was sharing a meal with my new dorm-mates in the DUC dining hall. In the manner of college freshmen everywhere, we were discussing current events. It was Sept. 12, 2001, less than 28 hours after the attacks, when I heard my first 9/11 conspiracy theory.

A friend was arguing that the plane that had crashed in Pennsylvania the previous day had been shot down by the U.S. military. His theory was not that the jet had been destroyed as part of some larger nefarious government plot, as some would later claim, but that it had been shot down to prevent another target from being hit. Furthermore, he argued, the Bush administration would never be able to admit this, because the public would never accept that the American government would order an American plane, over American airspace, with American passengers, to be shot from the sky.

To me, the government not only would have been justified, the American people would have very easily understood that it had been justified, not to mention the fact that such a secret would be impossible to keep. We had a friendly debate for about half an hour. The next day actual details of what happened on Flight 93 began to emerge, and my friend and I didn't broach the subject again.

Now that 10 years have passed, I found myself wondering: Whatever became of my friend's odd conspiracy theory? (For that matter, whatever became of him?) More generally, what has happened to the 9/11 conspiracy theory, in all its various and outrageous permutations, in the last decade? By tracing its history, and its responses to news events such as the Iraqi surge or the 2008 election or the death of Osama Bin Laden, would it be possible to show how and why conspiracy theories in general—or at least this one in particular—wax and wane?

Conspiracy theories thrive by appealing to existing hatred, paranoia, and uncertainty. The hatred can wither. The paranoia can crack. And the uncertainty can disappear. But the conspiracy theory lives or dies, prospers or fades, for reasons almost entirely unrelated to its actual content.

Consider: Within hours of the planes hitting the towers, the conspiracy theories had already begun to swirl. Many used them to pin blame on their favorite pre-existing bogeyman. Days after 9/11, for example, a rumor spread that 4,000 Jews had been warned about the attacks and failed to show up for their jobs at the Twin Towers. As outlined in Part 1 of this series, this story was debunked immediately and never gained traction in the West. Career paranoiacs in America, meanwhile, were pointing the finger squarely at the U.S. government. People like libertarian radio host Alex Jones and alternative media reporter Michael Ruppert came from different ends of the political spectrum, but they both "knew" instantly that powers more diabolical than al-Qaida were behind the attacks, specifically the all-pervasive New World Order and the oil-hungry, fascistic Bush administration.

Soon after the attacks, Ruppert and Jones both had begun to cultivate mythologies about what really happened that day. But in the immediate aftermath of 9/11, only a tiny segment of the American population, 8 percent according to one poll in early 2002, was inclined to believe that their government was lying to them about what happened that day.

In 2003 and 2004 the Iraq war and revelations about the misleading claims that led us into it opened more minds to the possibility that the government wasn't telling the "truth"—a word the conspiracists conscripted to their cause, calling themselves the "9/11 Truth Movement"—behind 9/11. In the meantime, inconsistencies in the official version of events and questions about Bush's dealings with the 9/11 Commission gave full-time conspiracists plenty of ammunition with which to work. Although most Americans still believed that the Bush administration was "mostly telling the truth," by early 2004 16 percent of the population believed it was "mostly lying" about how much it knew prior to the attacks—double the number from the same CBS poll two years prior. Mainstream Democratic politicians like Howard Dean started to tip-toe around the subject of Bush foreknowledge, while at least one member of Congress, Cynthia McKinney, embraced conspiracy theories outright. Fahrenheit 9/11, which obsessively reported Bush's connections to the Saudis and the Bin Laden family, was a smash hit, pulling in more money at the box office than any previous documentary.

And in 2004, "truthers" found their intellectual apostle in an elderly professor of theology, David Ray Griffin. One year later, a film called Loose Change was released on the Internet, and by the end of 2006 it had been viewed tens of millions of times. Part 2 shows how over the course of the four years, from the start of the Iraq war to when it reached its lowest point in terms of both public support and as a military campaign, a new pool of potential adherents was created from which these conspiracists could pull their ranks. By mid-2006, one in three respondents would tell pollsters that they believed the government either orchestrated the attacks or allowed them to happen in order to go to war in the Middle East.

Around this time books, conventions, and movies about the 9/11 conspiracy finally started to garner attention from more mainstream outlets. With attention came scrutiny. Yet even though most of the principal contentions behind the movement were proved to be false—most famously, by 100-year-old engineering journal Popular Mechanics—as Part 3 illustrates, the full-time conspiracy buffs doubled down. Instead of admitting mistakes and exploring more realistic premises, the loudest conspiracists started accusing anyone who would question their findings of complicity in the cover-up.

And then there was the role of anti-Bush sentiment. As Bush became a lamer and lamer duck, Bush hatred subsided, and its pool of potential adherents dwindled, the movement became prone to infighting and purges. Soon there was a circular finger-pointing squad, as detailed in Part 4. Some younger leaders, like Loose Change director Dylan Avery, were driven away by the cynicism and intense paranoia of the conspiracists around 2007, while others, like British peace activist Charlie Veitch, were accused of being government spies after renouncing their beliefs.

At that point, however, the movement had already begun its decline. By 2009, with the first-ever African-American president having taken office, the number of Americans who said that Bush let 9/11 happen in order to go to war in the Middle East was at 14 percent. (Because the wording of questions about responsibility for 9/11 has changed over the years, getting a consistent measure of the public's view is difficult. But in September 2007, a Zogby poll found that 26.5 percent of Americans believed "certain elements in the US government knew the attacks were coming but consciously let them proceed for various political, military and economic motives," and 4.6 percent more said that members of the government actively aided in the attacks.) In another poll in 2010, only 12 percent of Americans said they did not believe Osama Bin Laden had carried out the 9/11 attacks.

Ten years after 9/11, the 9/11 conspiracy theories are receding to where they started: on the fringe. Yet one of the primary drivers of their popularity—mistrust in public institutions—remains high. After a decade of war and economic catastrophe, Americans are more distrustful of their government and the media than at any time in modern history. In the final installment of this series, I ask what this rising uncertainty means for the 9/11 conspiracy theory, or for other such theories. And I check back with my old college friend.

Part 1: Where did 9/11 conspiracies come from?

Do you remember where you were when you heard a 9/11 conspiracy theory for the first time? Email us at slateconspiracy@gmail.com or share your story in the comments below and we'll compile the most interesting notes in an epilogue to this series.