It is known that Al-Qaeda had suicide operatives who hijacked American Airlines Flight 11 as well as United Airlines Flight 175. This led to the fall of the twin towers of the World Trade Center as well as the American Airlines Flight 77 which flew into Pentagon the same day, a few minutes apart from the other flights. The result was fire and impact as well as the chaos that resulted, not to mention the large-scale fall of property and loss of lives that resulted from these impacts. While the Twin Towers collapsed, resulting in the death of several thousands of people, the Pentagon was impacted as well; there was a fourth plane that was also targeted towards Pentagon, which was a strike that was averted when the passengers gained control of the flight and did not let it meet its target.
The conspiracy theories that are rife with these attacks include one theory about how knowledge about these forthcoming attacks was known, but that the higher authorities deliberately ignored such knowledge. Pre attack warnings must have existed, but probably adequate communication was not made between the intelligence communities and law enforcement authorities. This lack of communication between the agencies led to several accusations of bureaucratic inertia. Several laws were later passed as a result of the accusations that the US administration received when they were unable to thwart such attacks.
There were several sources that backed up such views such as Purdue University study carried out by several scientists, articles that were released in different magazines as well as new media that also issued similar broadcasts. Even government investigations later revealed that the FBI and other institutes did lack certain vigilance which support such accusations made against the US government regarding the 9/11 attacks in 2001.