(CNN) — When this century began, you could pull up to the airport 20 minutes before a domestic flight in the United States and stroll straight over to your gate. Perhaps your partner would come through security to wave you goodbye. You might not have a photo ID in your carry-on, but you could have blades and liquids.

Back in 2001, Sean O’Keefe, now a professor at Syracuse University and former chair of aerospace and defense company Airbus, was deputy director of the Office of Management and Budget in the George W. Bush administration.

“At the White House, I was a member of the National Council Security team,” he told CNN Travel. He and his colleagues had been briefed on the al Qaeda terrorist group and understood the threat it posed, “but at the same time our imaginations simply did not give us the capacity to think that something like