Private security experts say the airport screeners would be hard-pressed to stop sophisticated terrorists.

The F.B.I. also pointed out that any such pardon of the "currently incarcerated terrorists would likely return committed, experienced, sophisticated and hardened terrorists to the clandestine movement."

Even with faster deployment, the new breed of sophisticated terrorists make successful commando operations more difficult - or worse, irrelevant.

Even then, sophisticated terrorists will always look for new tactics to evade detection.

And a technically sophisticated terrorist can make them.

The greatest value of such primitive conspirators as the Miami men is as bait for more sophisticated terrorists who might try to use them as cannon fodder, he said.

Still, American officials believe that the nearly simultaneous bombings made it clear that the attack was the work of sophisticated terrorists, and would have required many months of planning.

Even so, the feat points to yet another avenue that sophisticated terrorists might take to threaten unprotected civilians.

A sophisticated terrorist could shield a bomb in a radiation-blocking material like lead.

But the evidence against Mr. Yousef suggests he is a more sophisticated terrorist, officials say.