Even as bits of plane and passenger were fished from the sea, even as families of victims clung to faint hopes at an airport hotel, even as people from the president on down urged everyone not to jump to conclusions, many Americans made the leap anyway:

It's got to be a bomb.

If the crash of TWA Flight 800 turns out to have been planned, it will complete a stunning trilogy of terrorism in the United States that began with the World Trade Center bombing in 1993 and was topped by the attack on the Oklahoma City federal building last year.

Those tragedies each spurred new demands for tighter security, quick arrests and prosecutions, and served as reminders that the United States no longer is immune from the sort of maniacs who once stuck to European capitals and Middle Eastern hot spots.

What is unclear is whether Americans will sacrifice some freedoms for more security, or merely resign themselves to the fact that political terrorism is here to join the local landscape of drive-by shootings, armor-piercing bullets, church burnings, serial killings, workplace massacres and crack attacks.

``We just don't feel safe in this country any more,'' said Bruce Blythe, a psychologist and head of Atlanta-based International Crisis Management Inc., which sells security and psychological services to private companies hit by violence or disasters.

Even though its cause was not immediately clear, the crash electrified the security-conscious Olympic Games in Atlanta and sent shudders through air travelers nationwide. Shares in security firms spiked higher on Wall Street.

Todd Sermershein of Salt Lake City, who was flying from Cleveland to Minneapolis on a business trip, was typical of those who just reflexively assumed the plane was blown out of the sky by a sinister force _ and that his jetliner might be next.

``My wife is scared to death,'' he said.

The mere fact that so many people thought the crash was the latest chapter in the country's recent struggle with international-style terrorism even affected the markets, said Allan Rones, a financial analyst who studies the security market for J.W. Charles Securities in Boca Raton, Fla.

``Any time this happens, Americans accept more security,'' he said after watching stocks in security firms he follows shoot up after the disaster. ``Whether or not it's a terrorist thing, just the thought is enough.''

Blythe said his company, which worked with businesses hurt by the Oklahoma blast, immediately got calls from airlines inquiring about his services.

``Why is that so?'' he said. ``It's because we've had a pattern of terrorism in this country."

Like the Oklahoma City bombing, the TWA crash raised the same queasy fears that nobody is safe anywhere from a nut with a cause.

``Once that message is conveyed through the media with its powerful images, the message becomes very powerful,'' said John P. Wilson, a Cleveland State University psychologist specializing in post-traumatic stress syndrome who has worked in Oklahoma City, Bosnia and post-war Kuwait.

The crash triggered the usual deluge of claims of responsibility, tips on suspects, missile sightings and other post-disaster confusion that adds to the national anxiety.

``There will be all sorts of leads that will go nowhere,'' said Henry J. DePippo, a former federal prosecutor who was the senior trial counsel in the World Trade Center case.

The crash already has brought a deluge of demands for tighter airport security in U.S. airports, which is much more lax than those in European capitals where terrorist attacks are as familiar as post office shooting sprees are here.

``We live in a different era than we did 10 years ago,'' Wilson said. ``Surely, people will tolerate much stricter and rigorous procedures.''

Not necessarily, said Blythe, noting that in this era of anti-government militia movements there are people opposed to, say, bans on assault weapons.

Still, the terrorist attacks that have most affected American civilians all have led to intensified efforts to toughen security, chase criminals abroad, pass new laws _ all part of what politicians say is common sense, but which psychologists call part of a society's necessary bid to wrest control of itself from the terrorists.

``It's important to have this sense that things are being done,'' said Ann Kaiser Stearns, a psychology professor and author of ``Living Through Personal Crisis.''

When suspects were found in the Oklahoma City and World Trade Center buildings, there was almost a collective sigh of national relief. But the culprits who bombed a Pan Am flight that crashed in Lockerbie, Scotland, in 1988 remain at large.

``The problem is not coming up with the suspects, it's coming up with the proof and getting the people before the court,'' said Joseph W. Dellapenna, a Villanova University law school professor and expert on transnational litigation.

Just catching somebody can bring a traumatic episode to an end.

``That is part of the healing: that justice be sought,'' Kaiser Stearns said.