9/11 Conspiracy Theories Explained: Fact, Myths About What Really Happened

The deadliest terrorist attack on the U.S. soil took place on Sept. 11, 2001, when a series of coordinated attacks carried out by 19 militants killed almost 3,000 people and left more than 6,000 people injured. The attacks left the World Trade Center collapsed, the Pentagon burned and a field in Pennsylvania smoldered.

On Tuesday, thousands of 9/11 victims' relatives, survivors, rescuers, among other people, are expected to gather at the World Trade Center to remember the horrific day.

" Tribute in Light will once again illuminate the New York City skyline as a symbol of our endurance and as a powerful reminder of the strength we found on 9/11," President and CEO of 9/11 Memorial & Museum, Alice M. Greenwald, wrote on the memorial's website.

It's been 17 years since more than a dozen Al Qaeda terrorists hijacked four airplanes for carrying out the suicide attacks. Two of the planes targeted the twin towers of the World Trade Center (WTC) in New York City, the third plane hit the Pentagon just outside Washington, D.C., and the fourth plane crashed in a field in Pennsylvania.

The American Airlines Boeing 767, which was loaded with 20,000 gallons of jet fuel, crashed into the north tower of the WTC, burning a hole near the 80th floor of the 110-story skyscraper, instantly killing hundreds and trapping others in higher floors.

Just 18 minutes after the first plane crashed into the WTC, a second Boeing 767 — United Airlines Flight 175 — sliced into the south tower near the 60th floor. The collision caused a massive explosion that led to huge chunks of burning debris dropping down on surrounding buildings and onto the streets.

"The fires from the impacts were intensified by the planes’ burning jet fuel. They weakened the steel support trusses, which attached each of the floors to the buildings’ exterior walls. Along with the initial damage to the buildings’ structural columns, this ultimately caused both towers to collapse. The five other buildings in the WTC complex were also destroyed because of damage sustained when the Twin Towers fell," the description on the memorial site's website on what happened at WTC reads.

As millions watched the events unfold in New York, American Airlines Flight 77 crashed into the west side of the Pentagon military headquarters, killing 125 military personnel and civilians, along with all 64 people aboard the airliner.

A fourth California-bound plane — United Flight 93 — was hijacked about 40 minutes after leaving Newark Liberty International Airport in New Jersey. A group of passengers and flight attendants attempted to fight the four hijackers, who had taken control of the plane and were in the cockpit. They are suspected to have attacked the cockpit with a fire extinguisher. The plane flipped over and crashed in a rural field near Shanksville in western Pennsylvania.

While the plane's target still remains unclear, many believe it may have been the White House, the U.S. Capitol, and the Camp David presidential retreat in Maryland, or one of several nuclear power plants along the eastern seaboard.

Despite it being nearly two decades since the attack, the conspiracy theories that surfaced at the time of the tragedy still is alive. Here are some facts and myths about the worst attack.

Jet Fuel Can’t Melt Steel Beams: Theorists claimed that American Airlines Flight 11 and United Airlines Flight 175 wouldn’t have been able to produce enough heat to melt the steel beams of the buildings. They claimed that a planned explosion or military grade missile was used t bring down the Twin Towers. However, it is not always necessary for steel beams to melt for a structure to be damaged and destroyed.

Steel loses about 50 percent of its strength at 1,100 degrees Fahrenheit, Farid Alfawak-hiri, a senior engineer at the American Institute of Steel Construction, earlier told Popular Mechanics.

Holes on the Towers Aren’t Right: The American Airlines Flight 77 left behind two 75-foot and 12-foot holes in the Pentagon building. However, theorists claim that there was no way that a 125-foot jet made such small dents, raising concerns over a cover-up as some debris from the crash site may be missing.

“It was absolutely a plane, and I'll tell you why,” Allyn E. Kilsheimer, a structural engineer who was the first to arrive after the attack had said. “I saw the marks of the plane wing on the face of the building. I picked up parts of the plane with the airline markings on them. I held in my hand the tail section of the plane, and I found the black box … I held parts of uniforms from crew members in my hands, including body parts. Okay?”

Puffs of Dust Show Controlled Detonation: A book by Eric Hufschmid titled, "Painful Questions: An Analysis of the September 11th Attack" claimed the concrete clouds that gushed out of the buildings did not come only from the collapse but occurred due to explosions.

Seismic Spikes Hint at Explosion: A columnist on Prisonplanet.com, a website run by radio talk show host Alex Jones, claimed the seismic spikes that were recorded are "indisputable proof that massive explosions brought down" the towers. Each "sharp spike of short duration," Prisonplanet.com said, was consistent with a "demolition-style implosion."

While the report cited two seismologists Won-Young Kim and Arthur Lerner-Lam's finding supporting the theory, Lerner-Lam later said: "There is no scientific basis for the conclusion that explosions brought down the towers. That representation of our work is categorically incorrect and not in context."