WASHINGTON -- The three airplane crashes today into the New York World Trade Center and the Pentagon apparently were the work of a well-planned and executed terrorist conspiracy, with members who appeared to be trained in skirting airport security, senior FBI officials said.

Most likely, they said, the plot involved several terrorists on each airplane in the event the hijacked pilots refused to kill civilians on the ground.

At a news conference just after 10 a.m. PST, President Bush said that he believed it was a coordinated series of attacks, and said he had put the military and counter-terrorism networks on the highest possible alert for more assaults.

"Make no mistake: The United States will hunt down and punish those responsible for these cowardly acts," Bush said, adding that he was in touch with world leaders in an attempt to find out who was responsible.

Behind the scenes attention focused on Osama bin Laden, the exiled Saudi militant who in recent years has declared war on America. American authorities have been tracking Bin Laden for years, trying to bring him to justice for the twin bombings of U.S. embassies in East Africa in 1998 that killed 224 people.

Counter-terrorism officials were careful to say that they are looking at all options and possibilities. Officials in the White House Situation Room were reviewing intelligence cables from around the globe and FBI reports from within the United States, and looking at other Islamic extremist groups, "home-grown" hate groups and other potential terrorists.

Intelligence sources briefed members of Congress within hours of the first attack on the World Trade Center's southern tower and confirmed that the U.S. government considers the terrorist attacks as the work of some organized group.

Sen. Orrin G. Hatch (R-Utah) said that during the briefing, intelligence officials said the attacks bear the "signature" of Bin Laden, who oversees a vast network of Islamic extremists throughout the world.

"Who else would have the capability to coordinate the simultaneous hijacking of several aircraft?" said one FBI official who asked that his name not be used. A former FBI agent, John L. Martin, also said the coordinated attacks bore the fingerprints of Bin Laden because they "were extremely well organized and extremely secret."

Authorities believe that the terrorists had help from airport ground crews, that they chose cross-country flights because the planes would be heavily loaded with fuel and that their handlers are probably amazed that their plans so easily slipped past the U.S. intelligence apparatus.

"We're just amazed at the level of coordination this would have taken," said a senior official. "And the more extensive the conspiracy, the more opportunity U.S. agencies should have had in picking up some trace that this thing was going to occur.

"That's what's incredible," he added. "You're only as good as the intelligence you have, and in this case, intelligence failed us."

Another senior official, also speaking on condition of anonymity, predicted that the investigation will become the largest federal criminal case here or abroad. "First of all, you've got every place that these jets were hijacked from," he said. "You've got crime scenes.

"Undoubtedly, we will check for cars in the parking lots that may have been left. What happened at the gateways? How did they get through? What are on the passenger lists, the luggage lists?

"We will be screening, and if there [are] cameras available, we will look to see if there was more that one hijacker. There probably were four or five on each flight, maybe, and what are their connections with other passengers?

"Were the tickets purchased sequentially? What about cabdrivers? Who gave people rides to the airport? It's just an incredible myriad of leads to follow up on."

What most intrigues investigators so far is how the planes were made to crash into the World Trade Center towers, the Pentagon and outside Pittsburgh.

"It's so unusual that a pilot would bank and deliberately do this," said one investigator. "What resonates in my mind is this: Were the pilots incapacitated? And if they were, then the hijackers had to have somebody about to fly the planes themselves."

He also noted that the conspirators knew they they had to act fast, that once U.S. authorities realized that the country was under attack from the air, they would shut down all air traffic--just as the Federal Aviation Administration did.