They were smart, skilled and ruthless, the terrorists who rocked the United States yesterday with devastating air attacks that destroyed the World Trade Center and heavily damaged the Pentagon.

They were prepared to die for their cause. And that focused suspicion on hard-line anti-Americans acting in the name of Islam -- specifically, on Saudi Arabian-born millionaire Osama bin Laden.

Sheltered in Afghanistan by the hard-line Taliban regime, Mr. bin Laden is accused of orchestrating earlier assaults on U.S. targets, notably the near-simultaneous 1998 bombings of two embassies in Africa in which 224 people died.

And as the airwaves crackled with threats of U.S. retaliation, prospects looked good for a stepped-up war on terrorism -- possibly including assassination of alleged masterminds.

Taliban spokesmen denounced the attacks. One of them, Abdul Hai Muttmain, said it was "impossible" for Mr. bin Laden to have planned "such a big conspiracy."

But Utah Senator Orrin Hatch, the senior Republican on the Senate judiciary committee, said intelligence agents picked up communications among bin Laden followers about the attacks.

"They have an intercept of some information that included people associated with bin Laden who acknowledged a couple of targets were hit," he told Associated Press.

Mr. bin Laden has declared a "holy war" against the United States. The United Nations has imposed sanctions on Afghanistan in a bid to force it to close terrorist training camps said to be run by Mr. bin Laden. In 1998, he co-signed a fatwa (religious edict), saying it was the duty of every Muslim to kill Americans.

The targets and method -- possibly including suicide pilots -- suggested someone with a burning hatred for the U.S. and the ingenuity to mount a tightly co-ordinated operation, involving security breaches at three major airports.

"This is perhaps the most audacious terrorist attack that's ever taken place in the world," Chris Yates, an aviation expert at Jane's Transport in London, told AP. "It takes a logistics operation from the terror group involved that is second to none. Only a very small handful of terror groups is on that list . . . I would name at the top of the list Osama bin Laden."

Former Central Intelligence Agency director James Woolsey said the attackers may have had the support of an anti-U.S. government. "We need to very carefully look at all these terrorist operations . . . and see if there was any chance that there was state involvement behind it," he said.

Among nations, a prime suspect would be Saddam Hussein's Iraq -- an enemy of the United States since the Persian Gulf war.

Iraqi television welcomed the attacks as the "operation of the century" and added: "The American cowboy is reaping the fruits of his crimes against humanity." But the style of the attacks is radically different from anything Mr. Hussein has undertaken before, and he is not known as a major sponsor of international terrorism.

Officially, Washington held off blaming anyone. They may have been motivated by memories of the rush to blame Islamic fundamentalists for the 1995 bombing of a federal building in Oklahoma City. That attack, in which 168 were killed and more than 500 injured, proved to be the work of homegrown antigovernment extremists.

But unnamed officials were quoted as saying there were indications of a link to Mr. bin Laden.

He is also suspected of planning abortive attacks on a U.S. warship in Yemen last fall, where 17 sailors died. In 1999, Algerian Ahmed Ressam was caught taking explosives into the U.S. from Canada; investigators believed he spent time in bin Laden-run training camps.

Four of Mr. bin Laden's followers were convicted in June for their parts in the 1998 embassy bombings. A Manhattan federal judge planned to sentence one of them today in a courthouse near the World Trade Center.

That could be one motive for the attack. And the editor of the London-based Al-Quds al-Arabi newspaper, Abdel-Bari Atwan, said associates of Mr. bin Laden warned him recently that a "huge and unprecedented attack" would take place soon.

Commentators said several factors were key in analyzing the attacks:

Several terrorists slipped through security checks and boarded four aircraft: two in Boston, one in Newark and one in Washington.

They chose wide-bodied jets bound for the West Coast, carrying enough fuel to cause massive explosions.

One attacker on each aircraft was able to fly the plane. Commentators agreed no commercial pilot -- even with a gun to his head -- would aim an aircraft at a civilian target.

They chose targets highly symbolic of U.S. financial, military and political power. They were the Pentagon, the trade complex and possibly Camp David, the presidential retreat 130 kilometres south of where one of the planes crashed.

The missions involved certain death for those undertaking them. Those who die in holy wars are promised entry to paradise in the Koran, Islam's basic text.

The hijackers may have been only lightly armed. CNN said Barbara Olson, a commentator for the network, told her husband by cellphone from one of the airliners that the hijackers carried only knives and cardboard-cutters. They herded passengers and crew, including the pilot, into the rear of the aircraft, CNN quoted her as telling her husband.

Daniel Pipes, director of the Middle East Forum and a long-time observer of radical Islamic groups, said there is likely to be "more havoc yet. I hope this is a wakeup call for America and the world as to the kind of enemies we have."

He was among several commentators who called for a military-style campaign against terrorists and those who harbour them.

Janice Gross Stein, director of the Munk Centre for International Studies at the University of Toronto, predicted U.S. policy will toughen, perhaps to include selective assassination of accused terrorists.