Fireproofing failures -- rather the impact of the plane crashes -- probably caused the World Trade Center towers to quickly collapse, architects and engineers told a federal panel today.

"The insulation is going to turn out to be the root cause," said James G. Quintiere, a professor at University of Maryland's Fire Protection Engineering Department who analyzed the fireproofing in the two towers.

Neither tower, he found, had fireproofing thick enough to withstand the fire's blast furnace intensity for two hours, which is considered the minimum needed for those on the upper floors to escape the towers. "A two-hour fire resistance is right on the ragged edge," Quintiere said.

The North Tower, which had 1 1/2-inch-thick fireproofing, fell in 104 minutes, and the South Tower, with its 3/4-inch-thick fireproofing, collapsed in 56 minutes.

The findings were disclosed at a hearing by the National Institute of Standards and Technology. The non-regulatory group, a part of the Commerce Department, plans to use the hearings and forthcoming investigations to improve building codes and high-rise tower designs.

Previous studies have suggested that thousands of gallons of jet fuel feeding a raging fire, rather than the impact of the airplanes, caused the towers to collapse.

While more than 40 architects, engineers and victims' relatives supported the panel, many speakers urged officials to speed up their investigation and examine the failure of the buildings' fireproofing material.

"There needs to be a change in the way buildings are inspected," said Roger G. Morse, an architect who specializes in forensic investigations of building disasters and has studied the World Trade Center. Typically, he said, inspectors examine fireproofing before construction is completed, and the work is often damaged in the final construction.

In the case of the World Trade Center, he said, construction workers apparently failed to apply asbestos properly to some beams 30 years ago. He found that asbestos had peeled off the core columns up to the 78th floor.

No asbestos was applied above the 78th floor, because federal regulations changed and prohibited its application. Instead, workers on the upper floors applied a non-asbestos fireproofing that was not as fire resistant. With better fireproofing, Morse said, the towers "probably would have held up a little longer."

Most experts said the problems are not unique to the World Trade Center, which was regulated by the Port Authority of New York and New Jersey. "It could happen anywhere," Morse said. "The situation at the Trade Center wasn't the worst that I've seen."

Several experts spoke of a national problem in high-rise buildings. "The fire service has seen a consistent weakening in fire safety," said Vincent Dunn, a retired New York City fire chief and fire safety consultant. He ran through a list of several New York City building fires where spray-on fireproofing did little to prevent the structure's destruction. He described climbing through these buildings after fires and found "nothing left up there but bent, warped, twisted steel. There's no spray-on [fireproofing] left."

Victims' relatives seemed relieved that someone finally was examining the engineering of the World Trade Center towers. But their outrage was palpable as experts talked of the fire safety failings.

"After nine months, after knocking on the doors of elected officials . . . we are finally here today," said Sally Regenhard, head of the Skyscraper Safety Campaign. "Apparently the problem of inadequacy in fireproofing is not something new."

NIST launched its 24-month study partly in response to the complaints of victims' groups. Victims' families were especially critical of a report released April 30 by the Federal Emergency Management Agency. They said the study did not include input from survivors of the attacks on the towers.

NIST officials expect to interview survivors and victims' relatives about the final moments before the towers' collapse. They also will examine victims' phone calls, said Mat Heyman, NIST's director of public and business affairs. The agency plans to keep a database to track the degradation of the towers from the time of impact of each airplane to the collapse of the towers.

Victims' families said that they hoped the hearing would prevent other families from suffering their same grief. "I can't tell you what it's like for a mother to see that building hit, knowing your child is in the building," said Mary Fetchet, who lost her 24-year-old son, Brad, in the World Trade Center. "We're all at risk today. . . . I do not want 3,000 people to lose their life and have it be in vain."