s the World Trade Center was being built in the late 1960's and early 1970's, scientists were learning that asbestos fibers in materials commonly used to fireproof steel beams could cause cancer in workers and bystanders who were intensively exposed to the fibers, especially around mines and manufacturing plants dealing with asbestos.

Anticipating a ban, the builders stopped using the materials by the time they reached the 40th floor of the north tower, the first one to go up.

Now some engineers and scientists — including at least one whose research supported an asbestos ban in New York City — are haunted by a troubling question: were the substitute materials as effective in protecting against fire as the asbestos-containing materials they replaced?

Asbestos, a fibrous, silicate mineral, was highly prized as a fireproofing component because of its high melting point and its resistance to chemical breakdown. It also conducts little heat and its fibers create strong, supple materials.

The question haunts those engineers and scientists, but not because they think asbestos insulation might have ultimately preserved the towers' steel beams and trusses, which buckled in Tuesday's infernos, causing the towers to collapse.

Virtually as one, experts on the development, testing and use of fireproofing materials say no standard treatment of the steel, asbestos or otherwise, could have averted the collapse of the towers in the extraordinarily hot and violent blaze.

But some wonder whether asbestos insulation might have kept the towers intact long enough for more people to have escaped. And more important, they say the disaster at the World Trade Center exposes a gap in their knowledge about many fireproofing materials.

While those materials are routinely tested under conditions typical of ordinary fires, their effectiveness at the much higher temperatures of last week's catastrophe is generally unknown. In the new world of domestic terrorism, some authorities say ignorance is no longer acceptable.

"Tests for very violent and very large-scale fires have not been done," said Dr. Yogesh Jaluria, a professor of mechanical and aerospace engineering at Rutgers.

Dr. Jaluria, who added that similar questions surrounded the widespread fires in Kuwait in the Persian Gulf war in 1991, said he was all but certain that asbestos would have made no difference in the attack on the towers. But he said that definitive data do not exist.

While no one believes that fireproofing or other technology is a substitute for preventing terrorism, some scientists say building designs and materials should be subjected to much more detailed analysis.

"The technology exists today to do a full-scale computer analysis of what would happen under these conditions that we saw on television," said Dr. R. Brady Williamson, an emeritus professor of engineering science at the University of California at Berkeley.

Before the trade towers are rebuilt — as some suggest they should be — such studies should be used to design buildings able to "withstand this scenario," Dr. Williamson said.

Guy F. Tozzoli, who as director of the world trade department for the Port Authority of New York and New Jersey from 1962 until 1987 was a prime mover of the twin towers project, said they were designed to survive being struck by a Boeing 707, the largest jet of the day. The studies envisioned a low-speed impact by a plane lost in fog, he said.

And, initially at least, each tower did survive the high-speed impact of a larger jet, a Boeing 767.

When it came to fireproofing, the Port Authority at first turned to a mixture containing about 20 percent asbestos that was sprayed onto steel beams, where it dried and formed an insulating layer intended to keep the temperature of the steel from rising above 1,100 degrees Fahrenheit.

"Steel loses about 50 percent of its strength at that temperature," and can begin to buckle under the load of a building, said Robert Berhinig, a section head in fire-resistive construction at Underwriters Laboratories in Northbrook, Ill., where such materials are tested.

The fireproofing material was manufactured by United States Mineral Products of Stanhope, N.J., under the trade name Blaze-Shield, said James Verhalen, who was then president of the company and is now its chairman. About 65 percent was "mineral wool" — essentially rock that was melted and spun into fibers — bound together by cementlike components.

But as the steel skeleton of the towers began to rise, cancer studies by Dr. Irving J. Selikoff, then director of the environmental sciences laboratory at the Mount Sinai School of Medicine, were signaling the beginning of the end for asbestos.

Tests that showed asbestos from construction sites was being blown into the air sealed its fate. In 1969, Mr. Tozzoli said, the Port Authority decided to switch to a substitute fireproofing not containing asbestos; the city banned the substance in construction in 1971. The project again turned to United States Mineral, which had developed a new product — also called Blaze-Shield — with the asbestos removed. In addition, more than half of the original, asbestos- containing material was later replaced, said Allen Morrison, a spokesman for the Port Authority.

Mr. Verhalen said the new product essentially contained more mineral wool and binders, but no asbestos.

"The fire tests at Underwriters Laboratories produced the same fire resistance as the asbestos-containing products," Mr. Verhalen said. Port Authority had the same results. "We tested the hell out of it," said Mr. Tozzoli, who saw the second jet collision from the toll plaza on the New Jersey side of the Holland Tunnel. The lack of asbestos fireproofing, he said, "had nothing to do with the collapse of the building."

But partly because the substitute materials have been tested largely at temperatures characteristic of ordinary office fires — involving paper and furniture — and not in such a cataclysm, others have their doubts.

"In retrospect, considering the recent events at the World Trade Center, I wonder if the performance characteristics of the replacement material were as good," said Dr. Arthur Langer, director of the environmental sciences laboratory at Brooklyn College.

Dr. Langer, whose measurements of asbestos in the air were important in Dr. Selikoff's work but who has since received financing from the asbestos industry for his own research, concedes that the answer may never be known. Although he still believes that the decision to change the materials was "a good one based on concerns over public health," Dr. Langer says the question of effectiveness against fire still haunts him.

Whatever their stance on that question, others suggest that the disaster may change the way building materials are chosen in the future.

The fire-protecting performance of asbestos compared with that of other materials is "a legitimate question," said Dr. Philip J. Landrigan, who succeeded Dr. Selikoff at Mount Sinai. (Dr. Selikoff died in 1992.) But Dr. Landrigan, a medical doctor whose formal title is chairman of the department of community and preventive medicine, said he was satisfied with research showing that the replacements were as good as those containing asbestos. And he said Dr. Selikoff's work had suggested that hundreds of thousands of people had died of cancer because of exposure to asbestos.

"The toll from asbestos has been truly massive," Dr. Landrigan said. "The difference, of course, is that it occurred one at a time."