"The Reagan administration set up three teams," Mann said. "Each team had about 50 federal officials. And it had a chief of staff, and it had a Cabinet member who was going to be the next president. And if nuclear war seemed imminent, these three teams would be sent out from Washington, to different locations around the country. And in succession, each one could take over the running of the country."

War Games and Reality

Key figures in the plan — who served as senior staffers for the Reagan, Clinton and both Bush administrations — participated in what for years were just exercises, war games, which they discussed recently with ABCNEWS' Nightline.

One of those senior staffers was serving in the White House on Sept. 11, 2001, when a version of the Armageddon plan was actually put into effect — most likely for the first time.

"On the morning of 9/11, the entire continuity-of-government program was activated," said Richard Clarke, former U.S. counterterrorism chief under Presidents Clinton and George W. Bush, and an ABCNEWS consultant. "Every federal agency was ordered, on the morning of 9/11, to activate an alternative command post, an alternative headquarters outside of Washington, D.C., and to staff it as soon as possible."

"[Vice President Dick] Cheney and [Secretary of Defense Donald] Rumsfeld had participated in this program," Mann said. "They knew all the details. So, what happens on Sept. 11? Immediately, Cheney goes off into the bunker and tells the president of the United States, 'Stay out of town.' … And Don Rumsfeld asks his deputy, Paul Wolfowitz, to get out of town. They begin to send out these teams of federal officials. This is really an echo of the continuity-of-government plan."

Secret Locations, Secret Codes

Formulated during the Cold War, the original plan dealt with the possibility of nuclear attack.

"Let's suppose that all of Washington, or all of the Atlantic Seaboard from, let's say, Connecticut down to Georgia, was wiped out — unfortunately, not an unlikely scenario if you had a true nuclear attack," said Edwin Meese, attorney general under President Reagan. "Or, let's suppose as far west as Chicago, all the large cities were wiped out. You have a real problem then, of even being able to communicate to find out if anybody is left in the Washington area that might be in that line of succession."

In their periodic drills, participants in the Armageddon plan practiced it in supersecret fashion.

"I remember one occasion where we got the call," Clarke said. "We had to go to Andrews Air Force Base and get on a plane and fly across the country. And then get off and run into a smaller plane. And that plane flew off into a desert location. And when the doors opened on the smaller plane, we were in the middle of a desert. Trucks eventually came and found us and drove us to a tent city. You know, this was in the early days of the program. A tent city in the middle of the desert — I had no idea where we were. I didn't know what state we were in. We spent a week there in tents, pretending that the United States government had been blown up. And we were it."

"It's as though you were living in a play," Clarke added. "You play-act. Everyone there play-acts that it's really happened. You can't go outside because of the radioactivity. You can't use the phones because they're not connected to anything."

"You're living it," said Kenneth Duberstein, a White House chief of staff under Reagan. "Back then, it was possible we were going to suffer a nuclear attack. But how do you start worrying about the food supply in the Midwest, whether the water is contaminated or not?"

The plan often called for coded communications.

"There's an elaborate system for the people in this network, first of all, to verify each other's identity," Clarke said. "That person on the other end has a certain password and information that they have to pass for us to believe that they're who they say they are."

To establish that the remote teams were, in fact, controlling the government, the plan often called for demonstrations to possible foes.

"Sometimes, you order U.S. forces to do something," Clarke said. "You say to the adversary in advance, 'I'm going to order our forces to do X. You will observe that. That's how you know that I'm in charge of U.S. forces.' "

9/11: ‘It’s Working’

The rehearsed mobilization may never have happened for a real scenario during the Cold War, but the lessons from all the drills came into play on 9/11.

"You had Cabinet officers who were dispatched around the country," Duberstein said. "You had people in Congress, the leadership of Congress, taken to other locations. And I said to myself, as we proceeded through the day, 'It's working.' All those days of patriotic duty were coming back and they were working."

Because President Bush was in Florida, Cheney initially stayed based at the White House. Bush also wanted to return to Washington, Clarke said, but instead was taken to strategic command headquarters in Nebraska, where, "He could have commanded U.S. forces, if he needed to."

If executive branch leaders and large numbers of congressmen had been killed in an attack on the United States, the plan could have gone further, officials suggest, perhaps even with non-elected leaders of the United States taking control and declaring martial law.

"I think in any war where Washington were destroyed, inevitably, there would be a period of, for lack of a better term, something like martial law," Clarke said. "The key here is, though, that the plans all call for going back to a normal three-branch system, as rapidly as possible."

Shadowy Forces?

The plan was so secret until recently, it's possible that on 9/11 even members of Congress were unaware of it.

"You wouldn't want hundreds of U.S. senators to know," Meese said. "You'd want the people who were concerned, like the speaker of the House and the president pro tem, because they were part of the plan."

Now that it's been reported, even members of the public know.

But out in the open, concerns have arisen that the plan might violate the constitutionally prescribed order of succession to the presidency — though advocates say the plan operates with the best intentions.

"I think the American people can know that the government is doing that which it should be doing, and that preparation for things that are unthinkable, and planning for contingencies that are unimaginable, is part of good government," said Sally Katzen, deputy director of the White House Office of Management and Budget under President Clinton.

"If people need to hear that there are particular programs that are achieving that objective, I suppose that's desirable," she added. "But for them to think that there's something conspiratorial, or that there's something underhanded about this, or that there's something circumventing the Constitution, would be a great disservice. This is done, I think, by the book — as it should be."