NEW YORK -- In a city full of famous buildings, the twin towers of the World Trade Center outdid them all - massive glass-and-steel sculptures that appeared to float above the tip of lower Manhattan.
At 110 floors each, the boxlike pillars looked simple in form, but they were sturdy architectural marvels that provided a home for 1,200 businesses, many of them involved in international trade.
First imagined in the early 1960s as part of an urban renewal project, the first buildings 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. At the 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 another 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.
FBI evidence in that case included documents from one conspirator, who wrote that the bombing was meant to demoralize the enemy by ''blowing up the towers that constitute the pillars of their civilization.''
Tuesday's tragedy clearly had that effect on Lewis Eisenberg, 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.''
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