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Chicago — The World Trade Center, a symbol of American economic might, survived one terrorist attack in 1993. It was designed to withstand the impact of a jet, but both its towers collapsed Tuesday morning after planes rammed into them.

The structural engineer who designed the towers said as recently as last week that their steel columns could remain standing if they were hit by a 707.

Plumes of smoke pour from the World Trade Center buildings in New York after planes crashed into the upper floors of the 110-story buildings. The Empire State building, seen in the foreground, was the scene of a plane crash in 1945. It remained intact and reopened for business within months.

Les Robertson, the Trade Center's structural engineer, spoke last week at a conference on tall buildings in Frankfurt, Germany. He was asked during a question-and-answer session what he had done to protect the twin towers from terrorist attacks, according to Joseph Burns, a principal at the Chicago firm of Thornton-Thomasetti Engineers.

Burns, who was present, said that Robertson said of the center, "I designed it for a 707 to smash into it."

Burns, whose firm did the structural engineering for the Petronas Twin Towers in Malaysia ? the world's tallest buildings ? said Robertson did not elaborate on the remark. Robertson could not be reached Tuesday.

Completed in 1972 and 1973, the 110-story twin towers were the fifth- and sixth-tallest buildings in the world. One World Trade Center, finished in 1972, was briefly after its construction the world's tallest building. The towers have been called "a monumental gate to New York and the United States."

They withstood the 1993 attack, when a bomb-laden van exploded, killing six people and injuring more than 1,000.

Closely spaced steel columns that ringed their perimeter held up the World Trade Center towers. Chicago's Aon Center (formerly the Amoco Building), completed in 1973, uses a similar support system, known to structural engineers as a "tube."

Shocked by the building's collapse, structural engineers pointed to fire as the likely cause of the structural failure.

"Fire melts steel," Burns said. In addition, he said, the impact of the plane could have severely damaged the building's sprinklers, allowing the fire to rage, despite fireproofing supposed to protect steel columns and beams.

"You never know in an explosion like that whether they (the sprinklers) get cut off," Burns said.

Tuesday's attack marked the second time that a plane has crashed into a New York City skyscraper, although the first incident was an accident.

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In 1945, a B-25 flying at 200 miles per hour slammed into the 78th and 79th floors of the Empire State Building, gouging an 18-by-20-foot hole 913 feet above the streets of Manhattan. The pilot, Lt. Col. William F. Smith Jr., had been heading from New York's LaGuardia Airport to Newark, N.J., when he became disoriented.

Fourteen people died in the crash and the fire that followed ? three people in the plane and 11 in what was then the world's tallest building.