Correction Appended

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 A. Salem, should be used, the informer said.

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. Four men are now on trial in Manhattan Federal Court in that attack.

Mr. Salem, a 43-year-old former Egyptian army officer, was used by the Government to penetrate a circle of Muslim extremists 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 trade center blast, he was feuding with the 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.

“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.,” Mr. Salem said Mr. Anticev had told him.

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.”

Mary Jo White, who, as the United States Attorney for the Southern District of New York is prosecuting defendants in two related bombing cases, declined yesterday to comment on the Salem allegations or any other aspect of the cases. An investigator close to the case who refused to be identified further said, “We wish he would have saved the world,” but called Mr. Salem’s claims “figments of his imagination.”

The transcripts, which are stamped “draft” and compiled from 70 tapes recorded secretly during the last two years by Mr. Salem, were turned over to defense lawyers in the second bombing case by the Government on Tuesday under a judge’s order barring lawyers from disseminating them. A large portion of the material was made available to The New York Times.

In a letter to Federal Judge Michael B. Mukasey, Andrew C. McCarthy, an assistant United States attorney, said that he had learned of the tapes while debriefing Mr. Salem and that the informer had then voluntarily turned them over. Other Salem tapes and transcripts were being withheld pending Government review, of “security and other issues,” Mr. McCarthy said.

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.

The transcripts do not make clear the extent to which Federal authorities knew that there was a plan to bomb the World Trade Center, merely that they knew that a bombing of some sort was being discussed. But Mr. Salem’s evident anguish at not being able to thwart the trade center blast is a recurrent theme in the transcripts. In one of the first numbered tapes, Mr. Salem is quoted as telling agent Floyd: “Since the bomb went off I feel terrible. I feel bad. I feel here is people who don’t listen.”

Ms. Floyd seems to commiserate, saying, “hey, I mean it wasn’t like you didn’t try and I didn’t try.”

In an apparent reference to Mr. Salem’s complaints about the supervisor, Agent Floyd adds, “You can’t force people to do the right thing.”

The investigator involved in the case who would not be quoted by name said that Mr. Salem may have been led to believe by the agents that they were blameless for any mistakes. It was a classic agent’s tactic, he said, to “blame the boss for all that’s bad and take credit for all the good things.”

In another point in the transcripts, Mr. Salem recounts a conversation he said he had with Mr. Anticev, saying, “I said, ‘Guys, now you saw this bomb went off and you both know that we could avoid that.’ ” At another point, Mr. Salem says, “You get paid, guys, to prevent problems like this from happening.”

Mr. Salem talks of the plan to substitute harmless powder for explosives during another conversation with agent Floyd. In that conversation, he recalls a previous discussion with Mr. Anticev.

“Do you deny,” Mr. Salem says he told the other agent, “your supervisor is the main reason of bombing the World Trade Center?” Mr. Salem said Mr. Anticev did not deny it. “We was handling the case perfectly well until the supervisor came and messed it up, upside down.”

The transcripts reflect an effort to keep Mr. Salem as an intelligence asset who would not have to go public or testify.

A police detective working with the F.B.I., Louis Napoli, assures Mr. Salem in one conversation, “We can give you total immunity towards prosecution, towards, ah, ah, testifying.” But he adds: “I still have to tell you that if you’re the only game in town in regards to the information,” then, he says, “you’ll have to testify.” Studied for Signs of Illegality

The transcripts are being closely studied by lawyers looking for signs that Mr. Salem and the law enforcement officials, in their zeal to gather evidence, may have crossed the legal line into entrapment, a charge that defense counsel have already raised.

But the transcripts show that the officials were concerned that by associating with bombing defendants awaiting trial in the Metropolitan Correctional Center, Mr. Salem might have been accused of spying on the defense.

In an undated conversation, Mr. Anticev tries to explain the perils.

“We’re not allowed to have any information regarding that,” he tells Mr. Salem. “That could jeopardize, you know, if you go see a lawyer, ah, you know, with the defendant’s friend or whatever like that, and you’re talking about things we’re not suppose to, ah, condone that. We’re not supposed to make people do that for us. That’s like sacred ground. You can’t be privileged, ah, you can’t know what’s being talked about at all.”

Mr. Salem seems to bridle. “I, I, I don’t think that’s right,” he says.

The agent insists: “Yeah, but that’s just a guideline. If that ever happened, ah, you can back and reported on the meeting between, ah, you know, Kunstler and Mohammad A. Elgabrown. Forget about it. I mean a lot of people ah the case can get thrown out. You understand?” The references were to the defense lawyer, Mr. Kunstler, and his client in the second bomb case, Ibrahim A. Elgabrowny.

Mr. Salem seems to reluctantly agree.

“They want you to have a hand in it,” Mr. Anticev goes on, “but they’re afraid that when you get that kind of, ah, too deep, like me, it’s almost like, especially with all this legal stuff going on right now.”

If it were just intelligence gathering, the agent says, “You can do anything you want. You could go crazy over there and have a good time. Do you know what I mean?”

The agent goes on: “But now that everything is going to court and there is legal stuff and it’s just, it’s just too hard. It’s just too tricky, if, this, you know. And then there’s the fact if you come by with the big information, he did this, ah, let me talk about this with the other people again.”

“O.K.,” Mr. Salem says. “All right. O.K.”