A NATION CHALLENGED: THE SITE; Engineers Have a Culprit in the Strange Collapse of 7 World Trade Center: Diesel Fuel – The New York Times

Investigation into collapse of 47-story 7 World Trade Center, a mystery since Sept 11, now centers on thousands of gallons of diesel fuel that were kept there to fuel electricity generators in event of power failure; one of generators was for mayor's emergency command bunker; similar back-up generators are used in buildings like hospitals and trading houses across the country, so definitive answer on Building 7 may be most important question facing investigators; photo; drawing (M)