An "Armageddon" program designed to ensure that the federal government would continue to function in the aftermath of a nuclear war was put into place during the Sept. 11, 2001, terrorist attacks.

According to ABC's "Nightline," which plans to report its findings tonight, every federal agency shifted its control to an alternate headquarters outside Washington. President Bush's decision to fly to Nebraska that day instead of returning to the White House, which drew some criticism at the time, was part of that plan, former counterterrorism official Richard A. Clarke said on the program.

"Nightline" expands on a book by James Mann that detailed the birth of the program, named "Continuity of Government," during the Reagan administration. Under the plan, if the United States were facing a nuclear attack, three teams of 50 federal officials would be sent from Washington to locations across the country -- each with a Cabinet member who was prepared to become president.

That is what happened on Sept. 11. "Questions were raised by talking heads about the president's courage or lack of it because he didn't return directly to Washington," "Nightline" anchor Ted Koppel said yesterday. "This was absolutely the Armageddon plan put into effect."

Clarke told the program: "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." The former administration official also said he has participated in regular exercises over the past 20 years in which he has "gone off into caves in mountains in remote locations and spent days on end in miserable conditions, pretending that the rest of the world had blown up, and going through the questions, going through the drill. . . . 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."

Mann, whose book "Rise of the Vulcans" was excerpted last month by Atlantic Monthly, reported that Richard B. Cheney, then a Wyoming congressman, and Donald H. Rumsfeld, then a drug industry executive, were heavily involved in shaping the program during the 1980s. Both men, who were also former White House chiefs of staff, participated in the mock disaster exercises, which included convoys of lead-lined trucks carrying sophisticated communications gear to the secret locations.

During the Sept. 11 attacks, Vice President Cheney and Defense Secretary Rumsfeld found themselves carrying out a plan they had designed two decades earlier for a very different kind of threat during the Cold War.

ABC confirmed that Rumsfeld ordered his deputy, Paul D. Wolfowitz, to move to an undisclosed location outside Washington. Cheney was similarly dispatched, as was House Speaker J. Dennis Hastert (R-Ill.), who under the Constitution is second in line for the presidency. Several Cabinet members, including Agriculture Secretary Ann M. Veneman and Interior Secretary Gale A. Norton, were also removed, said Mann, a former Los Angeles Times reporter.

Koppel said that most members of Congress will be surprised to learn the plan's details, although selected leaders have been briefed. He said he felt "reassured" by the plan because "it seems to make common sense. You want the executive branch thinking about how to restore some kind of order in what would be absolute chaos."

The Washington Post reported in 2002 that as part of the plan, Bush has dispatched a shadow government of about 100 senior civilian managers to live and work secretly outside Washington. These officials have been rotating in and out of one of two fortified locations along the East Coast, according to three officials with firsthand knowledge, the story said.

Former counterterrorism official Richard A. Clarke, left, describes the government's "Armageddon" program on ABC's "Nightline." The show expands on a book by James Mann, right, detailing the beginning of a plan for after a nuclear war.