A lot of celebrities question the official narrative of the 9/11 attacks on the United States.

Mark Ruffalo, Marion Cotillard, and Charlie Sheen have all expressed beliefs that the official government narrative of the 9/11 attacks are false. Getty Images

A number of famous people have floated conspiracy theories about the 9/11 attack on the World Trade Center. A lot of actors have signed the "Actors and Artists for 9/11 Truth" petition, which asked then-president Barack Obama "to authorize a new, truly independent, investigation to determine what really happened on 9/11."

There are a number of alternative theories about the attacks, which generally say that the US government is lying about how it happened and who was behind them. Many of these theories were popularized by "Loose Change," a hoax documentary that went viral after being uploaded to the now-defunct Google Videos site.

Here's what Mark Ruffalo had to say about the attacks in a 2007 interview:

"I saw the way they all came down. And I'm baffled. My first reaction was, 'buildings don't fall down like that.' I've done quite a bit of my own research ... The fact that the 9/11 investigation went from the moment the planes hit to the moment the buildings fell, and nothing before or after, I think, makes that investigation completely illegitimate. If you’re going to do a crime investigation, you have to find motive. We didn’t follow that. It was quickly pushed away, obviously. There was no evidence — it's the biggest crime scene. And [Henry] Kissinger in charge of it makes it slightly dubious. Who knows? None of us know what happened for real. But I’m totally and completely behind reopening that particular... Where is that money? Follow the money, guys!”

Another is Martin Sheen. Here's what he said about it, according to HuffPost:

"There are obviously a lot of unanswered questions, let me leave it that way, that are very, very disturbing. The key to that is Building 7 and how that came down under very, very suspicious circumstances."

Martin Sheen's son, Charlie Sheen, was reportedly set to star with Woody Harrelson in a movie called "September Morn," which questioned the official narrative behind 9/11. The movie never materialized. But here's what Charlie Sheen said in an interview with Alex Jones, the host and founder of Infowars, a conspiracy theory media site, according to a transcript from CNN:

"I saw the south tower hit live, that famous wide shot where it disappears behind the building and then we see the tremendous fireball. And there was just — there was a feeling that it just didn't look, how do I say this, it didn't look like any commercial jetliner I've flown on any time in my life. And then when the buildings came down, later on that day, I was with my brother and I said, 'Hey, call me insane, but did it sort of look like those buildings came down in a — in a controlled demolition?' "Show us this incredible maneuvering. Just show it to us. Just show us, you know, how this particular plane pulled off these maneuvers. What was it, a 270 degree turn at 500 miles per hour, descending 7,000 feet in two and a half minutes, skimming across tree tops the last 500 meters off the ground?"

The "The Mary Tyler Moore Show," "Elf," and "Up" actor Ed Asner also supported the false "controlled demolition" theory. He narrated the 15-minute program titled "Architects and Engineers – Solving the Mystery of Building 7." Here's what he said in a podcast interview:

"My bottom line on all of this is that this country — which is the greatest, strongest country that ever existed in the world, in terms of power — supposedly had a defense that could not be penetrated all these years. But all of that was eradicated by nineteen Saudi Arabians, supposedly. Some of whom didn’t even know how to fly."

Here, you can see his comments synced up to the lips of his character from "Up."

Rosie O'Donnell, on "The View," said she wasn't sure about government involvement, and argued that the physics were impossible unless there were explosives involved.

"I do believe that it's the first time in history that fire has ever melted steel. I do believe that it defies physics that World Trade Center tower 7 — building 7, which collapsed in on itself—it is impossible for a building to fall the way it fell without explosives being involved. World Trade Center 7. World Trade [Center] 1 and 2 got hit by planes — 7, miraculously, the first time in history, steel was melted by fire. It is physically impossible."

Willie Nelson, also on Alex Jones's show, questioned the government narrative behind the 9/11 attacks.

"I saw those towers fall and I've seen an implosion in Las Vegas, there's too much similarities between the two. And I saw the building fall that didn't get hit by nothing ... So, how naive are we, you know, what do they think we'll go for?"

In the same interview where she questioned whether Americans really landed on the moon, Marion Cotillard told the French TV program "Paris Premiere" that there was something fishy about the collapse of the Twin Towers.

"We see other towers of the same kind being hit by planes. Are they burned? [There] was a tower, I believe it was in Spain, which burned for 24 hours. It never collapsed. None of these towers collapsed. And there [in New York ], in a few minutes, the whole thing collapsed. "It was a money-sucker because they were finished, it seems to me, by 1973, and to re-cable all that, to bring up-to-date all the technology and everything, it was a lot more expensive, that work, than destroying them."

Cotillard later told Access Hollywood that the comments were "taken out of context."

"At no point did I intend to contest the horrific attacks of September 11, 2001, one of the most tragic days in all of history," she said. "Nonetheless, I sincerely regret if my comments offended or hurt anyone."