The US stopped hijackings in the 1970s by quizzing passengers before boarding.
El Al has been quite successful quizzing passengers before boarding (in one case screener checked bag of a single woman going to vacation in Israel, found a bomb that her boyfriend had planted to eliminate his pregnancy problem). The screener somehow thought something didn’t add up even though she was innocent (perhaps going on vacation without boyfriend seemed unusual).
As for September 11, 2001, FBI botched: local agents told HQ of a person who insisted on only practising maneuvering not takeoff and landing, but HQ did not act. Local agents jailed the person, so one flight had one less hijacker than the others.