Since their completion in the mid-1970s, the 110-story glass-and-steel twin towers of the World Trade Center had appeared to float like mirages above lower Manhattan, their peaks at times obscured by clouds.

It may in fact have been their grandeur that led to their destruction: A 1993 attempt to blow up the towers was the work of terrorists bent on demoralizing the country by felling the ''towers of the West,'' FBI documents show.

The boxlike pillars looked simple in form, but they were sturdy architectural marvels that provided a home for hundreds of businesses, many of them involved in international trade.

They were also awesome to look at, said Andy Thornley, who took his last glimpse of the famed skyline as he rode the bus to work Tuesday.

''I looked at the Manhattan skyline and thought there's no more beautiful place in the world. And now it's gone,'' said the 43-year-old insurance worker.

First imagined in the early 1960s as part of an urban renewal project, the first building in the $1.2 billion, 16-acre complex opened in 1970.

THE TWIN TOWERS were completed in 1976, immense in every detail - 43,000 windows, 99 elevators, 1,350 feet tall - and designed to be a critical hub for international trade. For a time, they were the tallest buildings in the world; until Tuesday, they remained the tallest in New York.

The buildings were designed to be especially sturdy, using load-bearing steel walls rather than the steel-cage construction typical of modern skyscrapers.

By the time the final building of the seven-building complex was completed in 1988, the center had lured scores of businesses, including commodity exchanges, major investment firms, banks, law firms and a hotel.

The center was fully rented out when the towers collapsed Tuesday. Roughly 50,000 people worked in the towers; the complex, which included an observation deck and a number of other tourist attractions, drew an additional 90,000 visitors each day, according to the Port Authority of New York and New Jersey, which runs the complex.

While it thrived as an international business hub, it also had become a clear target for terrorists.

On Feb. 23, 1993, bombs exploded in a parking garage beneath the center, killing six people and injuring 1,000. Six Islamic militants were convicted in the bombing and sentenced to life behind bars.

THAT BOMBING WAS designed to topple the two towers like dominoes, kill 250,000 people and convince Americans they were at war, according to a Secret Service agent who helped interview Ramzi Yousef, the alleged bombing mastermind.

FBI evidence in that case also included a document calling for destruction of the ''towers of the West.'' The unsigned statement, found in the home of convicted terrorist El-Sayyid Nosair, said the bombing was meant to demoralize the enemy by ''blowing up the towers that constitute the pillars of their civilization.''

The document continued: ''In this way God's enemies will keep busy rebuilding their fortress and refurbishing their morale ... (They) will at that moment find themselves in a state of extreme psychic weakness as a result of what they see all around, because the forces they were relying upon will be in ruins and total destruction.''

Tuesday's tragedy clearly had that effect on Lewis Eisenberg, the chairman of the Port Authority of New York and New Jersey, who said he was ''devastated beyond belief.

''I mean, in many respects this is significantly worse than Pearl Harbor, and we don't know who the enemy is. As Americans, we will pull together and do what's right.''

He said he was stunned: ''I just saw my two towers fall.''