Tapes May Complicate N.Y. Bomb Case : Trial: Informant’s revelations in recordings with FBI could make prosecutors’ task more difficult. Officials are now on the defensive.

WASHINGTON — Secret tapes suggesting that the FBI may have missed an opportunity to prevent the World Trade Center bombing have put federal law enforcement officials on the defensive and threatened to complicate the job of prosecuting the bombing suspects, officials acknowledged Thursday.

The tapes, made by government informant Emad Ali Salem, reflect conversations in which Salem complained to his FBI handlers that the agency had failed to act on his tips that a terrorist explosion was being planned at Manhattan’s 110-story trade center towers.

The blast last February killed six persons, injured more than 1,000 others and sent thousands more fleeing down darkened, smoke-filled stairways in the office complex. The explosion, which blew a five-story-deep crater in the underground parking garage, has been termed “the most destructive terrorist act in U.S. history” by federal prosecutors.

Taken as a whole, the tapes pose “an absolute nightmare for prosecutors,” said Richard Ben-Veniste, a Washington attorney and former associate Watergate prosecutor. Officials, while declining to elaborate for attribution, acknowledged that the development poses a problem.

Ben-Veniste and others said that the tapes show a distrustful relationship between the government and an informant whose veracity it must support when the cases come to trial. Otherwise, his value as a witness--perhaps the government’s most important one--will be badly damaged.

The taped conversations became public knowledge this week when a federal judge presiding over the trial of suspects in the case released transcripts to defense attorneys. The documents were then leaked to several New York news organizations.

In one conversation published by the New York Times, Salem--a former bodyguard for Egyptian cleric Sheik Omar Abdul Rahman, who is suspected of inspiring the bombing--said that law enforcement officials once planned to have him thwart the bombing by substituting harmless powder for the explosives, then called it off.

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Federal officials, while confirming the accuracy of selected transcripts, were so chagrined by the contents that they refused on-the-record comments. They also said that they did not wish to anger a federal judge who gave the documents to defense lawyers.

One high-ranking law enforcement officer claimed, however, that Salem’s remarks were exaggerated and “taken out of context.” The government lacked information before the bombing that was sufficiently specific to head it off, the source insisted.

“There’s no way the FBI would be in receipt of information that there was going to be some terrorist action and not act upon it,” the source declared. “That’s ridiculous.”

But government officials acknowledged that the tapes seriously complicate the prosecution’s tasks. One official said, for example, that by challenging Salem’s claim on the tapes that he gave the FBI advance knowledge of the bombing, the government raises questions about Salem’s overall credibility.

The same point was made by Harry C. Batchelder Jr., a former prosecutor and New York attorney who once represented Rahman. In an interview, Batchelder said that defense attorneys now may be able to show “inaction by the government and distrust of their own informant.”

The blind sheik, a spiritual leader for many suspects charged in the bombing and related acts, is among 15 defendants who are scheduled to be tried early next year for not only instigating the bombing but for plotting to blow up the United Nations headquarters and the Lincoln and Holland tunnels.

The FBI said that it foiled those attempts when it arrested a group of suspects last June at a makeshift Queens, N.Y., bomb factory. Salem, who secretly recorded many conversations with the suspects, is expected to be the government’s star witness in that case.

Although prosecutors had not planned to call Salem as a witness in the case of the four alleged bombers now on trial, some defense attorneys said they could summon him themselves if they decide he might help their case.

Salem, a former Egyptian army officer and body builder who gained the trust of the Muslim defendants, was equipped with a hidden microphone by the FBI. But without the knowledge of the agency, Salem also secretly recorded his conversations with agents when he began working with them in earnest in the weeks after last February’s bombing.

In one undated transcript he told an FBI agent that another agent had dissuaded him from complaining to FBI headquarters in Washington about the New York bureau’s alleged failure to prevent the trade center bombing.

The second agent, identified as Nancy Floyd on the tape, did not dispute Salem’s account, but responded: “Well, of course not, because they (New York officials) don’t want to get their butts chewed.”

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But a government source familiar with the case discounted the agents’ quotations, saying that they reflected agents’ trying not to upset a prime source of information.

“When you’re working an informant, the informant is never wrong,” this source said.

James M. Fox, who heads the New York FBI office, declined comment Thursday, as did Atty. Gen. Janet Reno, who was asked at her weekly press briefing whether the FBI had advance knowledge of the bombing.

“I didn’t know that it was going to happen but I would not comment on the prosecution of any matter . . . while the matter is pending in court,” she replied.