Law-enforcement officials were told that terrorists were building a bomb that was eventually used to blow up the World Trade Center. 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 A. Salem, should be used. The account, which is given in the transcript of hundreds of hours of tape recordings Mr. Salem secretly made of his talks with law-enforcement agents, portrays the authorities as in a far better position than previously known to foil the Feb. 26 bombing of New York City's tallest towers. The explosion left six people dead, more than 1,000 injured and damages in excess of half a billion dollars. 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. 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 ... does not dispute Mr. Salem's account, but rather, appears to agree with it. Other Salem tapes and transcripts were being withheld pending Government review, of "security and other issues." William M. Kunstler, a defense lawyer in the case, accused the Government this week of improper delay in handing over all the material. The transcripts he had seen, he said, "were filled with all sorts of Government misconduct." But citing the judge's order, he said he could not provide any details.