By successfully attacking the most prominent symbols of American power -- Wall Street and the Pentagon -- terrorists have wiped out any remaining illusions that America is safe from mass organized violence.

That realization alone will alter the way the U.S. approaches its role in the world, as well as the way Americans travel and do business at home and abroad.

The death toll from the hijacked jets' attacks that destroyed the World Trade Center in lower Manhattan, and damaged the Pentagon, was impossible to gauge immediately. But it could eclipse the loss of life the country suffered in the Japanese attack on Pearl Harbor, when more than 2,300 perished.

It wasn't immediately clear who was responsible for the attack, though official attention focused on Middle East terrorist Osama bin Laden and his organization. One U.S. official said intelligence agencies already had gathered "strong information" linking Mr. bin Laden to the attacks. If the bin Laden organization isn't directly responsible, U.S. officials suspect, it could have sprung from a network of Islamic terror groups he supports and finances. The gravity of the challenge to the country was summarized by Sen. John McCain, a Vietnam War veteran, who said: "These were not just crimes against the United States, they are acts of war."

Yet a war against terrorism is unlike a conventional war, and in some ways is far scarier. As a traumatized nation saw in gruesome detail on its television sets, terrorists attack civilians, not soldiers. And while the wars of the past century involved nation-states that could ultimately be defeated, a war against terrorism involves a less distinct enemy, whose defeat will be hard to ensure.

President Bush nearly promised armed response in his response to the tragedy. "America has stood down enemies before, and we will do so this time," he said in nationally televised address from the Oval Office. In a pointed warning to terrorists as well as to nations such as Afghanistan, which hosts Mr. bin Laden, the president declared: "We will make no distinction between the terrorists who committed these acts and those who harbored them."

Leaders of the House of Representatives and the Senate-shuttered yesterday amid the threat-plan to reconvene today in a special session to consider a bipartisan resolution condemning the terrorist attacks.

The sheer sophistication of the terrorists was remarkable. The FBI is operating on the assumption that there were multiple hijackers on each of the flights that struck New York and Washington. They apparently were armed with knives, and investigators believe that in at least two of the planes they "corralled and put in the back" the regular pilots, leading to the assumption they were experienced in handling jets. The FBI has been poring over airport security videos and flight manifests, and officials said they are finding strong leads to the identities of the hijackers from the names found there.

Last night, a law enforcement official said the FBI was seeking warrants to search a former residence of one of the hijackers in Daytona, Fla. The official added that airport video surveillance, as well as names on the manifests, suggested that the hijackers were of Arab nationality. In some cases they were armed with box cutters in addition to knives. One passenger, Barbara Olson, the wife of solicitor general Theodore Olson, telephoned the Justice Department in an attempt to reach her husband during one of the harrowing flights and said passengers were being held in the back of her plane before it smashed into the Pentagon.

In a clear sign of the operation's professional nature, a government official said the hijackers knew how to shut off the planes' transponders, which transmit airline flight number, speed and altitude. The official said it wasn't clear when the transponder in American Airlines Flight 11 from Boston, the first plane to strike the World Trade Center, was turned off, but it happened before it hit its target.

Meanwhile, average Americans far from the attack sites already are feeling the aftershocks. Many suddenly are worrying about a matter that had never previously occurred to them: the safety of their cities from coordinated attack. Shortly after the World Trade Center attack, Peggy Smith, an office administrator with the law firm of Conley Rose & Tayon, left her downtown Houston office clutching computer-tapes and copies of account data for safe-keeping. "This is the end of the world as we know it," she said. "The United States will never be the same."

Underscoring that sentiment, American F-16 fighter jets were scrambled and two aircraft carriers were dispatched, not to some distant foreign destination, but to protect the skies and seas around Washington and New York. For the first time ever, all airline flights were grounded across the country. Financial markets were closed.

The events occurred without any apparent warning, prompting immediate questions in Washington and elsewhere about a failure of U.S. intelligence. How did such a broad and coordinated attack on multiple sites occur without U.S. intelligence officials getting wind of it? How were so many commercial airplanes hijacked and diverted hundreds of miles out of their flight paths toward the nation's largest population centers? "Today our government failed the American people," said Rep. Curt Weldon, a Pennsylvania Republican.

Yet there were some hints of trouble that were, in retrospect, under-appreciated. A senior U.S. intelligence official who left the government earlier this year said that the joint FBI-CIA counter-terrorism center had been receiving what it considered solid intelligence during the past two months pointing to possible imminent attacks by Islamic extremists. The intelligence consisted of a noticeable uptick in communications activity among Islamic extremist groups.

Some officials believed, though, that the attacks were likely to occur overseas, as did recent attacks against American embassies in Africa and against the USS Cole in Yemen "We've known for the last two months that something was planned; just nobody knew where," the former senior official said.

At the same time, there had been heightened concern for several weeks about a possible attack on a military target in the Washington area, said a current U.S. official. For that reason, checkpoints at Fort Myer and Fort Belvoir, both in the Washington area, have been more strict. At the White House, even the cars of members of Congress have been checked for explosives, and there was a partial evacuation several weeks ago when a car suspected of carrying a bomb was spotted outside the executive mansion. "Who the hell would think they would fly airplanes?" one official asked.

There are multiple reasons to suspect Islamic extremists, which explains the immediate focus on Mr. bin Laden or liked-minded compatriots. Earlier this year, in a Manhattan courtroom only a short walk away from the World Trade Center, four of his followers were convicted on all charges in the 1998 bombings of two U.S. embassies in Africa. At one point, sentencing had been set for today, though that had been postponed.

At the same time, Sheik Omar Abdul Rahman, the spiritual leader of Al-Gama'a al-Islamiyya, Egypt's largest militant group, sits in a U.S. prison in Minnesota for his role in planning an earlier but failed attempt at terrorism in New York. His followers have been seething ever since he was convicted in 1995 for his role in a plot to stage a series of terrorist attacks in New York, and officials say he may have helped inspire a bombing in a parking garage of the same World Trade Center destroyed. "I've never forgotten about that blind sheik and what a symbol he was to radical Islamists," said Robert Blitzer, the FBI's former domestic terrorism chief. "This could be revenge."

Ties between Sheik Abdul Rahman's followers and the bin Laden world appear to have tightened. Just last month, the foreign minister of the Taliban, the Islamic organization that effectively runs Afghanistan and harbors Mr. bin Laden, suggested the U.S. could trade Sheik Abdul Rahman for several Western aid workers under arrest in Kabul.

The violence raging between Israel and Palestinians has given Islamic extremists more reason to be agitated at the U.S. Such anti-American entities as Iraq and the Hamas and Hezbollah extremist organizations have rallied to the side of the Palestinians, railing against both Israel and its American ally. In any event, the attacks themselves were so intricately planned and so vast in scope that they transcend any past terrorist action. Some experts speculated that the enormity of the plot could even point to the involvement of a hostile government, such as Iraq or Iran.

Many experts, though, agreed the simultaneous nature of the attacks and other trademarks pointed to the larger terror network run or somehow inspired by Mr. bin Laden.

The list of non-state actors even remotely capable of pulling off such an attack is quite small. The only group generally known for staging simultaneous, complex terrorist attacks is Al Qaeda, the loose organization led by Mr. bin Laden. The U.S. has indicted him for the two 1998 embassy bombings in East Africa, and U.S. officials say that evidence points convincingly to his involvement in the bombing last year of the USS Cole in the Yemeni port of Aden.

Other groups such as Hamas on the West Bank, or Hezbollah, in Lebanon, have staged truck bombings and suicide attacks in Israel and elsewhere across the region. But no one has ever pulled off a series of attacks of this magnitude. Nor, experts say, are either of those groups prone to targeting Americans, despite the fact that anger is now high toward the U.S. across the Arab world.

James Steinberg, former deputy national security adviser under President Clinton, said he believed that an attack of this size was likely the work of several groups within Mr. bin Laden's greater orbit. Of those, he listed the Algerian-based Armed Islamic Jihad and the Al-Gama'a al-Islamiyya, Egypt's largest militant group. Mr. bin Laden's Al Qaeda has been known for several years to be in close contact with operatives from a wide range of militant groups across North Africa and the Middle East.

Other terrorism experts said the attacks, in their sheer audacity, bore many trademarks of the bin Laden strategy. The African embassy bombings, one in Tanzania and the other in Kenya, occurred less than 10 minutes apart, while the attacks on the two World Trade Center towers happened within 18 minutes. The fact that the World Trade Center was at the center of the plot also points to the actors behind the 1993 Trade Center bombing, many of whom were later found to have had close ties to the bin Laden network, according to U.S. officials involved in the investigation.

Yet some experts also said the complexity of the operation made it unlikely that Al Qaeda could have pulled it off without help from other terrorist organizations more experienced at hijackings and the technical problems of overcoming airport security. Al Qaeda has been building ties with groups like Islamic Jihad, the Iranian-backed Palestinian terrorist group, which has threatened attacks against U.S. interests recently in response to Israeli use of U.S.-supplied fighters and helicopters on the West Bank.

One official noted that several of the crashed jets were laden with fuel, which would make it more difficult for hijackers who took control of the jets to maneuver them unless they were experienced or had some training at controlling large airliners. "If it turns out that bin Laden claims responsibility for these attacks, he couldn't have done it without help from professionals, like Islamic Jihad," said Robert Baer, a former CIA officer and Middle East specialist.

Certainly the attack would signal a frightening increase in Al Qaeda's deadly skills. Its previous attacks have used truck bombs and other crude devices. Other attacks linked to the group have been plagued by problems. More than a year before the bombing of the Cole, another attempt to bomb a U.S. warship failed when a boat carrying explosives sank. A Los Angeles airport bombing was thwarted altogether.

On a more ominous note, some former terrorism officials also speculated that the attacks may reveal that Mr. bin Laden now has a large and sophisticated domestic terror network operating within the U.S.

"It is not to be ruled out that there are tacticians, bomb-makers and plotters now fully active in the U.S., many of whom have been here for years," said Daniel Benjamin, a former counterterrorism official in the Clinton White House.

The diffuse and overlapping organization of today's terror groups became particularly clear after the aborted millennium plot in December 1999, when U.S. border agents arrested an Algerian crossing into Washington state with a trunk-load of explosives. Tentacles of that plot, which targeted the Los Angeles airport and other sites, extended from Canada and cities across the U.S. to actors in Algeria, Sudan, Egypt and Afghanistan.

In a bizarre twist, some experts suspect that the bin Laden organization may also have had a hand in a suicide bomb attack against Ahmed Shah Massoud on Sunday in northern Afghanistan. Mr. Massoud leads the opposition force fighting Afghanistan's Taliban leaders, who control about 90 percent of the country. The Taliban have given refuge to Mr. bin Laden since the mid-1990s. There are conflicting reports as to whether Mr. Massoud survived the blast.

For many Americans, a more tangible and bitter image of anti-American sentiment abroad will be the scenes of some abroad celebrating the terrorist attacks on Americans. In the West Bank, thousands of Palestinians took to the streets to herald the attacks and express their happiness. And in Sierra Leone, Pakistani members of a United Nations peacekeeping force were laughing, smiling and slapping hands at the mission headquarters in Freetown.

If the attack was launched by a non-state entity, choosing when and where to retaliate may not be easy.

After the bombing of two U.S. embassies in East Africa in 1998, President Clinton ordered cruise-missile strikes on a site where Mr. bin Laden and his top lieutenants were supposed to be meeting. As it turned out, the meeting had ended and the strikes came too late.

"The big question for everyone now is how much intelligence do we have? Do we have the kind of intelligence that we need?" said retired Gen. Dennis Reimer, former Army chief of staff and now head of the Oklahoma National Memorial Institute for the Prevention of Terrorism.

The U.S. could move more easily to punish any state that abetted Mr. bin Laden, especially Afghanistan, which has refused repeated demands to turn him over. A devastating military strike on the Taliban's headquarters could be one course.

Afghanistan's Taliban leaders were clearly very nervous about that possibility, denying Mr. bin Laden's involvement and calling for American "courts" to seek justice. A series of explosions last night in Kabul, the Afghan capital, apparently were part of internal fighting between the Taliban and its internal foes, and not part of any U.S. response to the terrorist attacks.