On Nov. 1, 1955, a United Airlines plane exploded and crashed in Colorado, killing 44 people. The cause turned out to be a bomb planted by a man who wanted to kill his mother, a passenger, for the insurance money.
Few Americans at the time would have suspected that politics _ or the willingness to use random mass murder to terrorize a nation _ would ever be behind such a disaster.
Times have changed. With the investigation of the crash of TWA Flight 800 barely under way, President Clinton said he wanted "to urge all the American people not to jump to unwarranted conclusions about the tragedy."
But many people already had jumped, assuming that the destruction of the Boeing 747 on Wednesday off the coast of Long Island was yet another act of terrorism in a country that used to think it was immune from such atrocities.
"It's changed because we're vulnerable," said Harvey Kushner, a criminal justice professor at Long Island University in New York, who studies and writes about terrorism.
Regardless of what caused the TWA crash, Kushner said, "We've been victims of terrorism, and the two oceans that separate us from the world are very short now. That's changed our psyche."
The result, said Robin Wagner-Pacifici, a sociologist at Swarthmore College in Pennsylvania, is that terrorism automatically enters our thinking as a possible cause of disaster.
"It's become one of the ways people have of trying to understand these kinds of tragedies, and that's a relatively new phenomenon," she said.
"Ten years ago they wouldn't have made that assumption. I think that terrorism up until then was seen as something that happened in other places, and terrorists were seen as something shadowy from other religious or ethnic origins."
Wagner-Pacifici, who has studied public attitudes toward terrorism here and in Italy, said Americans "were kind of slow in understanding the vulnerability of the U.S. to terrorists."
"People still want to resist thinking about how this is going to affect me on a daily basis," she said. "That sense of invulnerability is very slow to die."
There have been plenty of lessons, from the bombing of a Pan American jet over Scotland in 1988 to the World Trade Center bombing in 1993 to last year's truck bomb that destroyed the Alfred P. Murrah Federal Building in Oklahoma City.
Craig McDonald, an associate professor of criminal justice at the Rochester Institute of Technology, agreed that because of the attacks, "We are recognizing that we are a target and it can happen."
But that, he said, isn't enough.
"I think it's almost a two-stage thing," McDonald said. "Stage One is recognition that it could occur, and Stage Two is accepting that it's here and there are countermeasures that need to be employed. I don't think we have gone into a stage of acceptance that we are going to have to live a little differently."
McDonald said he hopes Americans will respond to the threat of terrorism without the stimulus of multiple tragedies.
"It's human nature that if it's not constantly in front of you, constantly on our minds, we'll tend to minimize it or become complacent again," McDonald said. "But I don't think we can afford to wait (for more attacks) because we're such a likely target."
He said Americans are heading for a future of stricter security measures and higher costs as airlines pass along the costs of protecting planes and passengers.
"And it's not just aviation," McDonald said. "I think even greater disaster could be at power plants, public buildings and, for shock value, even schools. I don't think we can gear up quickly enough, but we had better start."