Law-enforcement officials were told that terrorists were building a bomb that was eventually used to blow up the World Trade Center, and they planned to thwart the plotters by secretly substituting harmless powder for the explosives, an informer said after the blast.

The informer was to have helped the plotters build the bomb and supply the fake powder, but the plan was called off by an F. B. I. supervisor who had other ideas about how the informer, Emad Salem, should be used, the informer said.

The account, which is given in the transcript of hundreds of hours of tape recordings that Mr. Salem secretly made of his talks with law-enforcement agents, portrays the authorities as being in a far better position than previously known to foil the February 26th bombing of New York City's tallest towers.

The explosion left six people dead, more than a thousand people injured, and damages in excess of half-a-billion dollars. Four men are now on trial in Manhattan Federal Court [on charges of involvement] in that attack.

Mr. Salem, a 43-year-old former Egyptian Army officer, was used by the Government [of the United States] to penetrate a circle of Muslim extremists who are now charged in two bombing cases: the World Trade Center attack, and a foiled plot to destroy the United Nations, the Hudson River tunnels, and other New York City landmarks. He is the crucial witness in the second bombing case, but his work for the Government was erratic, and for months before the World Trade Center blast, he was feuding with th F.B.I.

Supervisor `Messed It Up'

After the bombing, he resumed his undercover work. In an undated transcript of a conversation from that period, Mr. Salem recounts a talk he had had earlier with an agent about an unnamed F. B. I. supervisor who, he said,

"came and messed it up." "He requested to meet me in the hotel,"

Mr. Salem says of the supervisor.

"He requested to make me to testify, and if he didn't push for that, we'll be going building the bomb with a phony powder, and grabbing the people who was involved in it. But since you, we didn't do that."

The transcript quotes Mr. Salem as saying that he wanted to complain to F. B. I. Headquarters in Washington about the Bureau's failure to stop the bombing, but was dissuaded by an agent identified as John Anticev.

Mr. Salem said Mr. Anticev had told him,

"He said, I don't think that the New York people would like the things out of the New York Office to go to Washington, D. C."

Another agent, identified as Nancy Floyd, does not dispute Mr. Salem's account, but rather, appears to agree with it, saying of the `New York people':

"Well, of course not, because they don't want to get their butts chewed."