(CNN) There are some popular -- and thoroughly debunked -- conspiracy theories that keep popping up after every mass shooting. Sandy Hook was fake. The Vegas shooting was carried out by multiple shooters. Columbine was a result of bullying.

Add to that, a new cottage industry of conspiracy theories that's already springing up around the high school massacre in Parkland, Florida.

Why do some people dream up such awful things at a time of great tragedy? Why do so many people believe them? And can anything be done to stop it?

Before we get to that, let's examine the conspiracy theories that will not go away.

Parkland

A woman cries during a candlelight vigil for the victims of the shooting at Marjory Stoneman Douglas High School

The conspiracy theory: Many of the students who survived the shooting at Marjory Stoneman Douglas High School in Parkland, Florida, have been very vocal about gun control.

That's led to accusations the students aren't really students at all. Memes and videos on YouTube claim some of the students are "actors" working for anti-gun groups who travel around the country to the sites of mass shootings. In fact, one of the top trending videos on YouTube this week alleged the same thing

The truth: That's just plain crazy, one of the outspoken kids told CNN. "I'm not a crisis actor," student David Hogg said on "AC360" Tuesday. "I'm someone who had to witness this and live through this and I continue to be having to do that." It prompted US Sen. Marco Rubio felt the need to come to the students' defense:

"Claiming some of the students on TV after Parkland are actors is the work of a disgusting group of idiots with no sense of decency," he tweeted

Las Vegas

The damaged windows on the 32nd floor room that was used by the shooter in the Mandalay Hotel

The conspiracy theory: The investigation into the October massacre that killed 58 people at a country music festival in Las Vegas ID'd Stephen Paddock as the only shooter. But don't tell the conspiracy theorists that. They claim there where were multiple shooters. So many kooky theories and untruths on the shooting flooded the internet that The investigation into the October massacre that killed 58 people at a country music festival in Las Vegas ID'd Stephen Paddock as the only shooter. But don't tell the conspiracy theorists that. They claim there where were multiple shooters. So many kooky theories and untruths on the shooting flooded the internet that YouTube changed its search algorithm to stop these conspiracy theories from bubbling to the top of searches.

The truth: While it's true that While it's true that investigators named two people as persons of interest in the early days of the investigtion, neither have been charged with a crime. One was Paddock's girlfriend who has since been publicly cleared in the case. The other was a business associate.

Sandy Hook

The conspiracy theory: The murders of 20 children and six adults at Sandy Hook Elementary School in December 2012 were shocking. But the fact there are people out there who believe it didn't happen is downright stupefying. The false narrative that Newtown, Connecticut, massacre was faked by the government is pushed by a number of conspiracy theorists, including Inforwars founder Alex Jones.

The truth: "They don't think anything bad ever happens, they don't think anyone ever gets hurt," Len Pozner, whose 6-year-old son Noah was killed in the shooting, "They don't think anything bad ever happens, they don't think anyone ever gets hurt," Len Pozner, whose 6-year-old son Noah was killed in the shooting, told CNN in 2016 . "They think whenever they see anything on the web or on television that is a crime or mass casualty event, that has to be a hoax." When Pozner began to see the hoax content appearing online, he worked to take down every video depicting false evidence, and created an organization called the HONR Networks to help him do so.

Columbine

Two students, Dylan Klebold, and Eric Harris, carrying guns and bombs, opened fire inside Columbine High School in Littleton, Colorado, on April 20, 1999

The conspiracy theory: Thirteen people were killed -- 12 students and a teacher -- on April 20, 1999, when a pair of students went on a shooting rampage at Columbine High School in Littleton, Colorado, one of the first school shootings to shock the nation. The conspiracy theory goes the shooters were part of a group called the Trench Coat Mafia who attacked their fellow students because they felt they had been bullied.

The truth: That's the That's the popular misconception that persists to this day, says journalist Dave Cullen, author of the book "Columbine." The killers weren't part of the Trench Coat Mafia (most members of that group had graduated years earlier); they weren't bullied by other students and that they didn't target popular jocks, African-Americans or any other group, Cullen says. A school shooting wasn't their initial intent, he told CNN in 2009. They wanted to bomb their school in an attack they hoped would make them more infamous than Oklahoma City bomber Timothy McVeigh.

The reason behind this misinformation spread

Snopes managing editor Brooke Binkowski, left

So why do so many people believe these theories?

Brooke Binkowski, managing editor of the fake news-busting website Snopes.com , said people who dream up and spread these untruths about tragedies probably never learned the most basic thing about compassion.

"These mass shootings -- who would fake these? You can't fake these horrible situations," said Binkowsi, a journalist whose career includes stints at Southern California Public Radio, CBS Radio and CNN. "I've covered their aftermaths myself and you cannot fake the smell of blood nor the way phones ring and ring and ring as people desperately try to talk to their loved ones, one last time."

JUST WATCHED Shooting survivor: I'm not a crisis actor Replay More Videos ... MUST WATCH Shooting survivor: I'm not a crisis actor 02:24

And the work of disentangling reality from a web of distortions dressed up as fact has become a persistent struggle for large segments of the population, a psychology professor said.

"People are constantly bombarded with information from so many different sources and it isn't always easy to work out what is true and what is false, and which sources are credible and which are not," Karen Douglas, a professor of social psychology at the University of Kent in the UK told CNN in 2016.

"Tens of millions of people believe in conspiracy theories," Dartmouth political science professor Brendan Nyhan told CNN. "There's a group of people that believe in a lot of them, but there's a much broader group that is willing to endorse them in certain cases. It's not a reflection of mental illness or pathology. It's a common thing that otherwise smart and well-informed people do."

While conspiracy theories aren't anything new, what is new is the rate of proliferation and speed at which the dark whispers turn to public pronouncements, thanks to social media and the internet.

"Someone like Alex Jones (of the Infowars website) can amplify the most effective conspiracy theories and spread them to a large audience," Nyhan said. "Sean Hannity, too. We've seen memes jump from the fever swamp online to Hannity and Alex Jones and even Donald Trump's Twitter feed very quickly -- people like that can harvest the conspiracy theories that they think are most interesting or entertaining or shocking."

How they can be stopped

The best way to combat this, said Binkowski, is by constant debunking, but also by pointing a finger at people with big media platforms who spread this garbage.

"We are debunking them as fast as we can. I am also aggressively calling out the people who are pushing these stories as part of my strategy," she said. "Not just because I find it disgusting, but also because when others see how fast they fold when confronted with facts, I think it helps discredit those pushing things that should be discredited."