''Even though Building 7 didn't get much attention in the media immediately, within the structural engineering community, it's considered to be much more important to understand,'' said William F. Baker, a partner in charge of structural engineering at the architectural firm Skidmore, Owings & Merrill. ''They say, 'We know what happened at 1 and 2, but why did 7 come down?' ''

Engineers said that here and across the country, diesel-powered generators are used in buildings like hospitals and trading houses, where avoiding power outages is crucial. Partly for that reason, Jonathan Barnett said, a definitive answer to the question of what happened in 7 World Trade Center is perhaps the most important question facing investigators.

''It's just like when you investigate a plane crash,'' said Dr. Barnett, a professor of fire protection engineering at the Worcester Polytechnic Institute. ''If we find a weakness in the building or a deficiency in the building that causes that collapse, we then want to find that weakness in other buildings and fix it.''

In many ways, 7 World Trade Center, built and owned by Silverstein Properties, was structurally similar to its towering cousins across Vesey Street to the south. The weight of the building was supported by a relatively tight cluster of steel columns around the center of each floor and a palisade of columns around the outside, in the building's facade.

Sprayed on the steel, almost like imitation snow in holiday decorations, was a layer of fireproofing material, generally less than an inch thick. Although the fireproofing was intended to withstand ordinary fires for at least two hours, experts said buildings the size of 7 World Trade Center that are treated with such coatings have never collapsed in a fire of any duration.

Most of three other buildings in the complex, 4, 5 and 6 World Trade, stood despite suffering damage of all kinds, including fire.

Still, experts concede, in a hellish day, 7 World Trade might have sustained structural injuries never envisioned in fire codes. That day began with flaming pieces of steel and aluminum and, horribly, human bodies raining around the building.

With the collapse of both towers by 10:30 a.m., larger pieces of the twin towers had smashed parts of 7 World Trade and set whole clusters of floors ablaze. An hour later, the Fire Department was forced to abandon its last efforts to save the building as it burned like a giant torch. It fell in the late afternoon, hampering rescue efforts and hurling its beams into the ground like red-hot spears.

Within the building, the diesel tanks were surrounded by fireproofed enclosures. But some experts said that like the jet fuel in the twin towers, the diesel fuel could have played a role in the collapse of 7 World Trade.

''If the enclosures were damaged, then yes, this would be enough fuel to explain why the building collapsed,'' Dr. Barnett said.

Dr. Barnett and Mr. Baker are part of an assessment team organized by the American Society of Civil Engineers and the Federal Emergency Management Agency to examine the performance of several buildings during the attacks. If further studies of the debris confirm the findings of extremely high temperature, Dr. Barnett said, ''the smoking gun would be the fuel.''

The work, which began last month at Seven World Trade Center, reflects both the adaptability of steel-framed towers and the extraordinary importance of fail-safe computer and telephone systems for the brokerage industry. According to many real estate experts, no company has ever made such extensive alterations to a new office building in Manhattan.

Salomon had tried to avoid the trouble and expense of alteration work by designing an office building, in partnership with a developer, from the ground up. But in late 1987, after the stock-market crash, Salomon withdrew as the co-owner and principal tenant of a project planned for Columbus Circle in midtown Manhattan.

The termination of that agreement left Salomon with an after-tax charge of $51 million and put the firm under intense pressure to find new headquarters space before its lease at One New York Plaza, in downtown Manhattan, expired in 1990. It no longer had time to shape the blueprints of a project; instead, it needed to find an existing building or one under construction that could be fitted for its high-technology operations in about two years.

After studying more than 50 options throughout the New York region, Salomon signed a 20-year lease for 22 floors - each spanning nearly an acre - at Seven World Trade Center, an office tower that has been largely vacant since Silverstein Properties completed it two years ago.

''We really had a time constraint,'' explained Gedale B. Horowitz, a senior executive director of Salomon. ''And we were driven very much by technology. We had to find a building that could accommodate our needs, including major-sized trading floors.''

Much of the new electrical, air-conditioning and mechanical equipment will serve three double-height trading floors. To create the extra height, workers are removing most of three existing floors, using jackhammers to demolish concrete slabs and torches to remove steel decking and girders beneath the concrete.

After the girders are cut into sections small enough to fit into a construction elevator they will be sold as scrap for about 4 cents a pound.