It was the unthinkable: A terrorist attempted a suicide bombing at a D.C. Metro station. Two hours later, three Cabinet secretaries were killed in a car accident after attending an event at the National Press Club.

At the same time, airports in Atlanta, Baltimore, Boston and Chicago experienced electronic interference in their air traffic control systems. And intelligence reports indicated an imminent threat of a major terrorist attack in Washington.

With that made-up catastrophic scenario in hand, more than 2,500 federal employees from 45 agencies left the Washington area Tuesday night and Wednesday. They headed to more than 100 secret sites to participate in the nation's first test of how the government could continue operating in the face of a massive terrorist attack.

"There has never been an exercise of this nature or of this magnitude, even during the Cold War," said Michael Brown, the undersecretary for emergency preparedness and response at the Homeland Security Department. "Our attempt was to get people focused on plans in the event of another 9/11. You don't want to wait until disaster hits."

The government-wide exercise, dubbed "Forward Challenge '04," was run by the Federal Emergency Management Agency. The goal was to train federal employees to operate as a shadow government away from their work sites. Reynold N. Hoover, FEMA's director of national security coordination, said some federal offices across the country also participated in the two-day trial run.

In the exercise, the president makes the decision to notify agencies to implement their "continuity of operations" (COOP) plans. At 9 p.m. Tuesday, a federal employee acting as the president made that notification after the three members of his Cabinet -- not identified to reporters -- were killed.

By Wednesday, under the fake scenario, hackers had coordinated an attack on government computers and penetrated several power grid computer systems, severe weather was reported and the D.C. water supply "was at risk."

Homeland Security Secretary Tom Ridge and several top aides went by helicopters yesterday to a secure facility outside Washington to observe the exercise. Speaking to reporters by videoconference from the undisclosed location, Ridge said the exercise, which officials planned for a year, had gone well, but he gave no details.

Homeland Security officials would not provide details on what federal employees did at the secret sites outside Washington. They said only that the employees practiced everything from the basics of getting into locked buildings on the sites and turning on the lights in an emergency to communicating by phone and computer with other agencies.

Exercise participants also worked on implementing succession and delegation of authority plans, and tested their ability to access records and databases.

Some participants stayed overnight at the undisclosed sites, and officials compared the sleeping facilities to motel rooms.

Brown said members of Congress and the judicial system were involved in COOP planning, but no one from Congress or the courts participated. State and local officials also were not part of yesterday's exercise.

The participants did not have to face the realistic obstacle of trying to leave Washington along with thousands of other residents during a major terrorism incident.

"Any exercise like this will have some artificiality," Brown said.

In March, the General Accounting Office found that none of the 23 major departments and agencies it studied had fully complied with a six-year-old presidential directive to develop emergency plans in accordance with guidelines from FEMA. The report concluded that the agencies had not developed adequate plans for the continuation of essential government services during emergencies.

The study also found that FEMA, now part of the Department of Homeland Security, fell short on oversight of the plans and that its guidance for agencies lacked detail.

Yesterday, Brown called the GAO report "off base" and praised the performance of the agencies that were part of the exercise.

Asked whether any lessons had been learned from the exercise, FEMA's Hoover said it was "premature" to discuss deficiencies because the exercise was ongoing. An after-action report will be done by early July, he said. The costs of the exercise, funded by the individual participating agencies, were not available yesterday.

Secretary Tom Ridge of the Homeland Security Department greets the controllers of Forward Challenge '04.