"This is a declaration of war," Johnson said. "You don't go in for a tie here. If these guys want to cross the line this way, so be it. But so will we. We can't go back now. If we don't act, the U.S. will be seen as unable to fight."

But John L. Martin, the former chief of the internal security section at the Department of Justice, urged restraint.

"Any kind of retaliation must be very restrained, and methodical, deliberate and accurate," Martin said. "Or else it's going to worsen the situation."

Rep. Dana Rohrabacher (R-Huntington Beach) called the attacks a "day of infamy" for the nation's intelligence community.

"For the national security apparatus to have missed this is the biggest intelligence blunder in our lifetime," Rohrabacher said. "The people we pay billions of dollars to have left us at the mercy" of international terrorists."

Brian Jenkins, a Rand Corp. expert on terrorism and international crime, said in Santa Monica: "We've seen elements of this event before. Thirty years ago, almost to the day, Sept. 6, 1970, four hijackings, one of which failed, involved the Popular Front for the Liberation of Palestine, who held hostages in the desert outside Jordan. It was a multiple coordinated hijacking.

"And we've had concerns that hijackers might crash into a major city. Algerian extremists in Marseilles [in] 1994 wanted refueling. French Intelligence, listening, feared they would take off and crash into Paris.

"Today, versus 30 years ago, the acts are large scale and often indiscriminate. The World Trade Center event today combines the two."

At UCLA, David Rapoport, editor of Terrorism and Political Violence, an academic journal, said there has been nothing like today's attack "in the history of terrorism."

Rapoport said the attack required "extensive planning by numerous individuals."

"The organization capable of this act has managed to elude our intelligence. We were not looking at the right organizations. And if we were, our failure is even greater."

Contributing to this report were Times staff writers Ricardo Alonso-Zaldivar, Geraldine Baum, Stephen Braun, Richard T. Cooper, Megan Garvey, Judy Pasternak, David G. Savage, Esther Schrader, David Willman, Robin Wright and Aaron Zitner in Washington; John J. Goldman and Thomas S. Mulligan in New York; Eric Slater in Chicago; Edward J. Boyer, Richard Lee Colvin, Thomas Curwen, Abigail Goldman, Ardith Hilliard, J. Michael Kennedy, Jeff Leeds, Hilary E. MacGregor, Eric Malnic, Richard E. Meyer, Diane Pucin, Mary Rourke, Tim Rutten, Henry Weinstein and Nona Yates in Los Angeles.