The upper floors of the World Trade Center's twin towers collapsed and toppled over because both airplane crashes turned the buildings into infernos, according to the head of the Maryland firm that cleaned up the last terrorist attack on U.S. soil.

J. Mark Loizeaux, president of Controlled Demolition Inc. in Phoenix, said that the large amounts of jet fuel that spilled from both airliners - loaded with enough fuel to fly across the country - probably ignited fires that softened steel supports and caused them to fail.

CDI, a 41-year-old firm known for pioneering the use of explosives in structural demolitions, took down the Alfred P. Murrah Federal Building in Oklahoma City after the terrorist bombing in 1995.

The firm also demolished the Kingdome in Seattle and Three Rivers Stadium in Pittsburgh, along with several housing projects in Baltimore.

Loizeaux said that skyscrapers are designed to survive plane crashes - and have survived them.

An Army Air Forces B-25 hit the Empire State Building in 1945, killing 14 people.

The twin 110-story towers of the World Trade Center were constructed in the 1960s and 1970s to survive the impact of a crash of a Boeing 707 commercial airliner, Loizeaux said.

And they did survive the initial impacts yesterday, he said.

But the impacts of the planes striking the buildings probably destroyed their fire protection systems, and the heat from the fire softened the towers' steel columns "like a piece of taffy would become soft in the sun," he said.

"They survived the crash, but the damage overloaded the remaining columns, and as the fire burned over time, it heated the remaining columns, making them ductile," he said.

Loizeaux said yesterday that most of his information about the attack was from television news reports, but he said he doesn't believe explosives were detonated after the trade center crashes.

"Based on what I have seen, and the information I have," he said, "this appears to be a reasonable explanation for what happened."