It's laughably unlikely.
1) We don't have guns everywhere in Europe. Indeed, the only people with guns are police officers and army officials.
2) 9/11 was blind luck and only worked because America was completely unprepared for that sort of thing, and even then only because of some unfortunate physics. The twin towers were built to withstand an impact from an aeroplane.
3) Al-Qaeda, as has been mentioned, is now extremely weak. Afghanistan and Iraq are both under American occupation, and all of the training camps have been shut down.
4) The threat level has been on "severe" for a good five years now.







