There is the remote possibility that Serbian terrorists were involved.
The original title of the 911 attack was Operation Bojinka.
There is some debate over the origin of this word. It is not clear whether it is a Serbian word or not. Catholic religious garb was found when Bojinka was uncovered. Ramzi Yousef's origins are unclear, though he is supposedly from the middle east, somewhere. He has reportedly now 'converted' to catholicism. He could have some as yet unrecognised Serbian ties.
Nikola Kavaja was a Serbian terrorist who hijacked a plane and planned to fly it into a tower in Belgrade.
Later he would claim Osama bin Laden stole his idea.
He was released from prison in 1997 and returned to Serbia where he set up a military training camp.
The FBI originally suspected Serbians for both the murder of Meir Kahane and for the WTC 93 bombing
Slobodan Milosevic reportedly forged a military alliance with Saddam Hussein, against NATO.
Saddam had a willing supply of jihadi idiots to use in any attack. He even pays bribes to suicide bombers families.
It would make a kind of sense for Serbians to get their own back on NATO for supporting the jihadi terrorists, + they loved Meir Kahane