The informer at the center of the Government's case in the plot to bomb New York City landmarks had a volatile relationship with his handlers, often quarreling with F.B.I. agents who used him to infiltrate a group of Muslim extremists who have been charged in the plot, according to transcripts of secretly taped conversations.

Then he added, still speaking to the agent: "You were informed. Everything is ready. The day and the time. Boom. Lock them up and that's that. That's why I feel so bad."

Federal officials have acknowledged in the past that they dropped Mr. Salem as an informer sometime before the trade center bombing over what they said was his reluctance to wear a body recorder, as well as other disagreements. They said he never provided detailed information of the attack in advance but that they began using his services again after the bombing and credited him with foiling the related but separate plot to bomb the United Nations, Holland and Lincoln tunnels and the Federal building housing the F.B.I. in Manhattan.

The case is expected to come to trial next year, perhaps shortly after the end of the related trial of four men charged with bombing the World Trade Center. As the most important witness, Mr. Salem is expected to be called upon to verify tapes he made of conversations with suspects and testify on his dealings with them.

Associates and lawyers of some of the defendants said that Mr. Salem appeared rather abruptly on the scene around the time of the Kahane killing and that they now suspect he was sent to infiltrate the circle around Mr. Nosair.

L aw-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 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.

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."

CORRECTION-DATE: October 29, 1993, Friday

CORRECTION:

An article yesterday about accounts of a plot to build a bomb that was eventually exploded at the World Trade Center referred imprecisely in some copies to what Federal officials knew about the plan before the blast. Transcripts of tapes made secretly by an informant, Emad A. Salem, quote him as saying he warned the Government that a bomb was being built. But the transcripts do not make clear the extent to which the Federal authorities knew that the target was the World Trade Center.