Yes, all four flight numbers involved in the September 11, 2001 terrorist attacks were permanently retired by American Airlines and United Airlines shortly after the tragedy. American Airlines Flight 11 (which hit the North Tower) and Flight 77 (which hit the Pentagon) were retired, as were United Airlines Flight 175 (South Tower) and Flight 93 (Pennsylvania). This is a standard industry practice out of respect for the victims and to avoid psychological distress for passengers and crew who might otherwise be assigned to those numbers. Airlines occasionally retire flight numbers after any major fatal crash, but the 9/11 retirements are the most globally recognized. Today, these routes still exist—for example, the morning flight from Boston to Los Angeles still departs daily—but they operate under completely different flight numbers, serving as a silent tribute to those lost in the attacks.