After the 9/11 attacks, what type of fiscal policies did the United States federal government impose on the airline industry?

Is the United States federal government responsible for high airline fares?

Since the airline industry is an oligopolistic market structure why do you suppose it would be unlikely for other airline carriers to increase their airline fares when one airline carrier increases their fares?

What is the timeframe in which the work hours caused a market failure in the airline industry? Was this before COVID-19?

Can you explain whether the United States federal government stepped in to fix this type of market failure?