The US government was informed about the 9/11 hijackers and the Boston city Marathon bombers etc. But, they get 99.99% false positives, so such information is basically worthless unless it results in conviction. Further the 9/11 hijackers used weapons no more deadly than what people are still allowed to bring on aircraft. If there where enough terrorists to matter they could have repeated 9/11 any time in the last 10 years. The most 'useful' security measure from 9/11 is a 'secure' door that locks which directly relates do another crashed aircraft making even that if anything a net loss. http://www.cnn.com/2015/03/26/europe/france-germanwings-plan...
That said, like the 'war on drugs' it has nothing to do with the actual 'threat' it's all just politics.