The organizers of the steel recovery effort will not say how many pieces in total have been set aside for future study, saying only that it is more than 100 from among the hundreds of thousands of World Trade Center steel parts. But without a doubt, the teams surveying have made some important discoveries.

The steel column identified last Wednesday by Ms. Bonilla, with its brackets, bolts and two pairs of winglike steel plates still attached, is a potentially critical discovery. It ran three stories from the 98th to the 101st floor on the exterior face of one tower, just above the zones struck by jets laden with fuel. Now that it has been found -- and spray-painted with the word ''Save'' in florescent orange paint -- tests can be conducted to determine whether heat or stress or some design or material flaw might have let it fail.

Another crumpled steel member set aside at a Keasbey, N.J., scrapyard has markings clearly showing that it ran on the east face of the north tower from the 92nd to the 95th floor, in the center of one impact zone. Pieces of steel have also been found that were apparently melted and vaporized not solely because of the heat of fires, but also because of a corrosive contaminant that was somehow released in the conflagrations. And unexpectedly cracked washers in crucial connections in the towers are being closely scrutinized.

The organized steel recovery effort is one piece of a three-tiered investigation. The American Society of Civil Engineers, in coordination with the Federal Emergency Management Agency, has set up a panel of engineering and fire experts who are assembling photographs, witness interviews and other data to try to determine why the towers collapsed. The effort, organized by the Structural Engineers Association of New York, is the team's primary source of raw steel evidence from the site.

Volunteers from the Structural Engineers Association visited ground zero first in early October and identified a handful of steel samples they wanted to save. But it was not until the start of November that three separate teams started to visit the steel yards on a regular basis. Since then, about 40 visits have been made to the three recycling yards and the Fresh Kills Landfill in Staten Island, where some steel is also being sent.

Much of the effort is focused on trying to find steel that was at or above the floors rammed by the hijacked jets. The towers collapsed from the top down and the fires were concentrated in these areas, so the presumption is that the answer to why they fell down should be found by examining the steel from these spots.

When the towers were built, markings were inscribed on the steel columns to indicate their exact positions. But with the damage from the fire and collapse, many of those markings cannot be found. Instead, the engineers measure the width of the steel: the higher it is in the building, the thinner it is. For example, plates that are three-eighths of an inch thick -- used to connect columns -- were used only above the 80th floor.