This week's coordinated terror attacks on commercial and governmental sites in New York and Washington have stunned terrorism experts in their scope and sophistication -- and prompted dire warnings that more could be in store for American citizens.
Daniel Pipes, who has written frequently on terrorism, said he believes Tuesday's attacks are only Phase One of a massive assault against the United States.
"There's an incredible amount of venom in the air against the United States," said Pipes, who is director of the Middle East Forum, a think tank.
Both towers of the World Trade Center collapsed Tuesday, and portions of the Pentagon were destroyed when commercial planes were hijacked and crashed into them.
Another plane crashed outside Pittsburgh, and a fourth crashed into the Pentagon outside Washington, causing part of the building to collapse.
The attacks have been called the worst against the United States since the attack on Pearl Harbor, which prompted the United States to join World War II.
Several Palestinian groups immediately claimed responsibility for the attacks, according to media reports, but it was initially unclear who exactly was responsible.
Suspicion is focusing primarily on Osama bin Laden, the Saudi billionaire who is believed to have masterminded the 1998 attacks on U.S. embassies in Africa and other terrorist incidents around the world.
An official of the ruling Taliban party in Afghanistan, where bin Laden is based, released a statement condemning Tuesday's attacks.
"We want to tell the American children that Afghanistan feels your pain," said Mullah Abdul Salam Zaeef, the Taliban ambassador to Pakistan. "We hope the courts find justice."
While his people celebrated and distributed candy in the streets, Palestinian Authority President Yasser Arafat condemned the attacks and sent condolences to President Bush.
Bush canceled an appearance in Florida and asked for a moment of silence soon after the attacks in New York.
"I've ordered that the full resources of the federal government go to help the victims and their families, and to conduct a full-scale investigation to hunt down and to find those folks who committed this act," Bush said. "Terrorism against our nation will not stand."
Terrorism expert Steve Emerson said he did not believe some of the Islamic groups being mentioned as potential masterminds behind the attacks had the ability to mount such a widespread effort.
"No one ever thought a coordinated attack was possible," he said. "They have never demonstrated the capability before."
Emerson called the series of attacks "unfathomable."
"There has been a fundamental mistake looking at this as a criminal problem, when in fact it is a military problem," said Pipes of the Middle East Forum. "You don't deploy policemen and lawyers. You deploy soldiers."
Pipes said it will be easy to determine what group is responsible for the attacks, because few have the capability. He said he hoped this would be an educational lesson for the United States, but was more cautious than some who believed it would be a turning point for U.S. counterterrorism efforts.
"If today doesn't have a chemical or biological component, that's what we have to look for in the future," Pipes said.
In an 1997 article, Emerson said he believed Muslim fundamentalist groups were preparing for a wide-scale attack against the United States.
"In fact, I would say that the infrastructure now exists to carry off 20 simultaneous World Trade Center-type bombings across the United States," Emerson warned in the interview with Middle East Quarterly. "And as chemical, biological and even nuclear weapons become available to them, the threat becomes ever more ominous.
"Just because someone holding a gun to your head doesn't pull the trigger it should not be understood as the threat not existing," he said. "It would be suicidal to permit our national security to depend on the good will or rationality of radical fundamentalists."
David Makovsky, senior fellow at the Washington Institute for Near East Policy, said the United States should be humble about making predictions regarding who is responsible.
"If, indeed, this is some sort of Islamic terrorism, the short-term impact may be giving the Israeli government more space in its fight against the threats that it faces daily," said Makovsky, former editor of the Jerusalem Post.
"There is no doubt that, at least in the short term, Americans will have a greater appreciation for what Israel has been going through on a daily basis for the last year.''
JTA correspondent Sharon Samber in Washington, D.C., contributed to this report.