Last month, researchers at Fairleigh Dickinson University asked a number of Americans whether or not they believed in a variety of political conspiracy theories.

Did they think that it was definitely true, for example, that George W. Bush knew about the 9/11 attacks before they happened? That President Obama is hiding personal details from his background? That global warming is a myth?

Most people said that none of the six theories presented were definitely true — but the responses varied.

Only 30 percent of supporters of Donald Trump, for example, rejected all six of the theories, a lower figure than backers of any candidate still in the race at that point.

And no conspiracy theory had a higher percentage of people saying it was definitely true than one that is particularly salient this week: “As secretary of state, Hillary Clinton knew the U.S. Embassy in Benghazi was going to be attacked and did nothing to protect it.”

Twenty-three percent of respondents said that this idea was “definitely true” — including 44 percent of Republicans and fully half of Trump supporters.

By contrast, only 40 percent of Trump supporters thought Obama was hiding important information about his early life (i.e. his birthplace).

The release of the final report from the House Select Committee on Benghazi on Tuesday — the most thorough examination of the events related to the attacks in Benghazi on Sept. 11, 2012 — didn’t offer evidence that the above theory was true.

But that didn’t stop Trump’s special counsel, Michael Cohen, from tweeting an image that went a step further, accusing Clinton of having “murdered an ambassador” — referring to Chris Stevens.

That’s the disconnect on the Benghazi issue, one that’s been baked-in for months now.

A large chunk of Americans — mostly Republicans — believe that Clinton, as secretary of state, behaved improperly and the exhaustive House investigation was warranted. Another large chunk — mostly Democrats — believes that the attack was a tragedy and that the House investigation, stretched into the presidential election year, was motivated by politics.

A subset of that first population takes the idea further: Clinton willfully ignored the issue and is directly responsible for the deaths of Stevens and the three others.

Last October, when Clinton spent hours testifying before the select committee, The Post and ABC News asked Americans if they thought the committee’s efforts were focused on addressing real concerns or on taking out the Democratic presidential front-runner.

Republicans said it was the former; Democrats, the latter. (Supporters of Bernie Sanders were slightly less sympathetic to Clinton than other Democrats.)

The same partisan split existed when respondents were asked whether or not they approved of how Clinton was handling questions about the attacks.

The next month, a McClatchy-Marist poll asked whether or not the investigation should continue. Republicans said yes. Democrats said no.

That the investigation is already viewed strongly through a partisan lens isn’t necessarily bad news for Republicans.

The most politically significant revelation of the committee’s work was almost certainly that Clinton used a private email server while serving as secretary of state. That detail, spun off into its own investigation run by the FBI, arguably holds the most potential for damaging Clinton’s campaign.

But damage already has been done. Last October, Fox News asked in a survey if people thought Clinton had been honest about the State Department’s role in the Benghazi incident. Most Republicans said no; most Democrats, yes.

But 30 percent of Democrats said she hadn’t — perhaps a reflection of support for Sanders, but also probably because of a decline in Clinton’s perceived trustworthiness.

In March 2015, just as the email story was breaking, 73 percent of Democrats and Democrat-leaning independents viewed Clinton as honest and trustworthy in Post/ABC polling. A year later, that figure had dropped to 58 percent. Among independents, Clinton saw a decline of 17 points.

Part of those later numbers for Democrats may also be due to the contested primary, but it’s fair to assume that much of the drop was a function of the questions raised by the Benghazi investigation and the email revelations.

The House report likely will be yet another Clintonian Rorschach test. Most Democrats will see it as a nonissue. Most Republicans will see it as an indictment. The Trumpian conspiracy theorists will see it as something else entirely.

What it’s unlikely to do is change many minds, given that the concrete already has set.