Here at 911blogger is a very interesting and fruitful discussion about the backgrounds of the hijacker, which I would like to recap.

Despite studying for years, until nowadays I am not able to come to a stringent conclusion, how these attacks were practically fulfilled inside the planes. Therefore it is a good way to hold it simple: There were the alleged hijackers on board, which headed the planes into the buildings. Many hijackers were seen at the airports. With that "best" scenario in mind, this conclusion would be in line with the warnings of immanent attacks shorty before 911, the described intelligence-failures by Richard Clarke (source) and the excellent research by Kevin Fenton.

Radical, suicidal islamists are necessary to be in accordance to this "best" scenario. James Redfoord has already written about the "western" lifestyle of some hijackers (source). Much information is won by interviews of Daniel Hopsicker with the former American girl-friend of Atta, Amanda Keller. For example should Atta have been a fan of the  musicgroup "Beastie Boys" (source). Snowcash has replied, that the hijackers could have behaved in that way, because they would have known, that their sins had been "expunged" by their martyrdom on 911 (source). Nevertheless there is information, which makes it even more difficult, to believe this "best" scenario.

There are indications, that some hijackers knew, that they were protected by US-authorities. They did not hide their intentions. For example walked Atta into the US Department of Agriculture to tell, that bin Laden “would someday be known as the world’s greatest leader.”, to threatened an employee, Johnelle Bryant, to cut her throat and fly planes into "your friends’ building?”(source). This could be explained by stupidity, but following information shows conclusively the working-relationship between the alleged radical islamists and US-authorities: There was a canceled flight, which gives reason to the suspicion, that it was an avoided hijack.

United Airlines Flight 23

9/11, just after 09:19 am: “After the plane was boarded, United Airlines officials told passengers that [the flight] had been canceled. Three males traveling refused to disembark. The argument with a member of the flight crew became so heated that the crew member called airport security. But before security arrived, the men had vanished, said the source, who spoke on the condition of anonymity. (...) Evidence Indicates Plans for Hijacking Plane - Authorities will later check the men’s unclaimed baggage and find box-cutters, copies of the Koran, and al-Qaeda instruction sheets."(source (history commens))

Let´s assume, that these failed hijackers could flee from the airport without noticing, that the US-authorities helped them to escape. But what happens then? There is indication, that the same men tried to enter planes on more time just days after 9/11!

On September 13: "New York authorities take into custody ten people of Middle Eastern descent at JFK International and La Guardia Airports, reportedly fearing they intend to hijack aircraft and commit another suicidal terrorist attack on a US target. (...) Furthermore, several of the detained men “showed up at the airport with tickets for flights canceled on Tuesday [September 11] and tried to use them.” Investigators say they believe one of the men had been among a group of passengers that behaved suspiciously and became aggressive after their aircraft—United Airlines Flight 23—had its takeoff canceled on the morning of 9/11 (...) According to the Washington Post, the detained individuals are carrying knives and false identification." (source)

This cannot be explained anymore by stupidity. It is obvious, that the alleged hijackers knew, that they were protected. It is hard to understand, how the hijackers should have been on the on hand side radical islamists, full of hatred against the western world, on the other hand side partners of just these western authorities.