The Mossad/Israelis knew all about the plot. They had a whole unit of intelligence agents -- the "Art Students" -- here monitoring the progress of the hijackers. Watching to see if the US intelligence or law enforcement was developing any awareness of the coming attack. I don't know of any action they may have taken, as they monitored the progress of the plot, to shield the hijackers from discovery or suppress any discovered elements from leading to such discovery. But they knew, and watched, and waited, and did nothing, clearly in favor of the attack succeeding, as that would serve Israeli interests.
Israel is no "friend/ally" of the US. They are no one's "friend/ally". Quite the opposite. With their supremacist attitude -- a suicidal tribal attitude of contempt for anyone not "Israeli" -- they are everyone's adversary, having demonstrated that character for 5000 years. The tragic truth of the Israeli situation is that in the end, the real "existential threat" to Israel,.... is Israel.
I'm an American and a Jéw.