From the New York Times, February 2, 2002
"Perhaps the deepest mystery uncovered in the investigation involves
extremely thin bits of steel collected from the trade towers and from 7
World Trade Center, a 47-story high rise that also collapsed for
unknown reasons. The steel apparently melted away, but no fire in any of
the buildings was believed to be hot enough to melt steel outright.
"A preliminary analysis of the steel at Worcester Polytechnic
Institute using electron microscopes suggests that sulfur released
during the fires -- no one knows from where -- may have combined with
atoms in the steel to form compounds that melt at lower temperatures.
A Search for Clues In Towers' Collapse; Engineers Volunteer to Examine Steel Debris Taken to Scrapyards