It’s been 25 years since al-Qaida terrorists attacked the United States by hijacking four commercial airplanes and crashing them into the World Trade Center towers and the Pentagon.

A fourth plane crashed into a field in Shanksville, Pa..

Sept. 11, 2001 represented a new kind of national security threat, and a new national security architecture was created to confront it.

Before a live audience at NPR’s headquarters in Washington, D.C., Sources and Methods host Mary Louise Kelly discussed how that day changed the country and its spy agencies, with people who worked on the frontlines of government and intelligence.

That panel included Stephen Hadley, Jane Harman, John Mclaughlin and Michael Morell. At the time of the attacks, Hadley was serving as deputy national security adviser in the Bush White House. Jane Harman was a Democratic member of Congress representing California. John McLaughlin was deputy director of the Central Intelligence Agency. Michael Morell was also at the CIA and was President Bush’s CIA briefer that very day.

The following highlights from their conversation have been lightly edited and condensed for clarity.  

On their memories of the day:

John McLaughlin: I was running a staff meeting. We were talking about the budget, including the budget for counterterrorism. We were talking about the Balkans and some other problems we were struggling with at that moment. And the first plane hit. And like everyone else, my thought was, ‘Could be terrorism. But let’s see what is going on here.’

And of course, the second plane left no doubt in our minds. I thought, and I think everyone there thought, ‘We are under attack.’ The other thought was in my mind and the mind of many people would say, ‘OK, that’s it,’ because we had been anticipating a major attack. We did not know time, target or method. But we had been receiving that summer, a giant spike in reporting about terrorism, something that the director subsequently characterized as the lights were blinking red all summer. But at the end of the day, I did write on my computer, ‘Nothing will ever be the same.’

Stephen Hadley: I was in the Situation Room. Dick [Richard] Clarke, the White House counterterrorism chief at the time, had a group in the back room, trying to figure out what we knew, what happened, whether this might be the first of successive attacks, more coming.And then the White House evacuated, and we saw these people running down West Executive Avenue. So I asked the group in the Situation Room, ‘Do you want to leave? Or do you want to stay?’ Everybody said: ‘We’ve got to stay and help the president manage this crisis,’ so we did.I think one of the things I remember most was Norm [Norman] Mineta, then secretary of transportation, getting thousands of airplanes in the air over this country down onto the ground, because we did not know how many might have been taken over by terrorists and become a weapon against this country.