COOPERATION with "friendly" intelligence agencies was the established practice of U.S. embassies and the CIA abroad, and that included granting visas to known agents to conduct intelligence missions in the United States. But something about the request Ambassador George W. Landau received in late July 1976 from a Paraguayan government official in Asuncion aroused his suspicions.

The official, a top aide to Paraguayan President Alfredo Stroessner, assured Landau that Chilean President Augusto Pinochet himself was asking for a favor. The official said he needed visas immideately for two Chilean Army officers using Paraguayan passports to travel from Asuncion to Washington on an intelligence mission. The mission, he said, had been cleared with the CIA station in Santiago and the two men would be in touch with CIA Deputy Director Vernon Walters in Washington.

Ambassador Landau, according to his later testimony, issued the visas for the two men the next morning. But his suspicions led him to take two precautions: He had the agents' false Paraguayan passports photographed, and he sent the photographs to CIA headquarters with a full account of the affair -- just in case the Chilean agents were lying about why they were going to Washington.

Landau's action was the first brush by a U.S. official with Chile's secret operations leading up to the assassination of Orlando Letelier six weeks later. In the weeks preceding the assassination of the leftist former ambassador and foreign minister, a flurry of cables and official communications went back and forth between the U.S. Embassy in Asuncion, the State Department, the CIA and the Immigration and Naturalization Service concerning the two Chilean agents, whose real identities -- not learned until almost two years later -- were Michael Townley and Armando Fernandez, the Chilean secret police agents who led the operation to kill Letelier.

CIA Director George Bush and his deputy, Gen. Walters, were among those who personally received and acted on Landau's warning. The ambassador's cable, sent via a top secret State Department "back channel," went first to the office of Secretary of State Henry Kissinger.

All that raises a series of disturbing questions. U.S. officials at the highest levels knew in advance about Chile's undercover mission in Washington and possessed photos and passsport information. Was that information sufficient foreknowledge to have prevented the murders? Once the assassination occurred, was the information turned over immediately to the FBI by the persons and agencies possessing it?

The pictures and the advance information obtained by Landau and others ultimately provided the keys to solving the case. But, unlike fictional spy mysteries, all the pieces of the puzzle did not fall into place with the identification of the guilty. Instead, the U.S. agencies involved in the case imposed an extraordinary mantle of secrecy over the actions of U.S. officials before and after the assassination and over the records and files relating to those actions. Given the secrecy about the extent of U.S. government foreknowledge, the questions we raise can only be partially answered.

According to our reconstruction of events, the Letelier assassination was set in motion in late June 1976. Pinochet's intelligence service had received reports of Letelier's recent visit to Holland to lobby against a $63 million investment by a Dutch company in Chile and of confidential meetings in New York between Letelier and a prominent leader of Chile's centrist Christian Democratic Party. Congress had just cut off Chile's military aid because of human rights violations.

Over the next three months, Col. Manuel Contreras, chief of DINA, the Chilean secret police, dispatched five of his agents on four separate but interrelated missions to Washington to carry out the order to kill Letelier. Of the four missions that made up the Letelier assassination operation, at least two were detected by U.S. authorities.

The first operation got only as far as Asuncion. DINA agents Townley and Fernandez went there to obtain false Paraguayan passports from the Paraguayan intelligence service and proceed on to Washington. After days of delay, they received the passports under the false names of "Juan Williams" (Townley) and "Alejandro Romeral" (Fernandez).

According to Paraguayan intelligence chief Col. Benito Guanes, they said they needed the passports for a trip to the United States to buy weapons and intelligence equipment "for which [they said] they could count on cooperation from the CIA/USA."

Ambassador Landau was told the CIA was aware of the mission, but that it involved surveillance of Chilean Marxists who had infiltrated the U.S. offices of the Chilean copper corporation.

After issuing the visas and photographing the "Williams" and "Romeral" passports, Landau wrote a long top secret cable to CIA Deputy Director Walters, who he had been told would be meeting the two Chileans in Washington. That cable remains secret, but we learned some of its contents. In it Landau asked Walters to confirm that the Chilean intelligence mission had been worked out with the CIA. He also sent copies of the passports to the CIA via diplomatic pouch.

Meanwhile, Townley and Fernandez, unaware their pictures were now in the hands of the CIA but suspicious of the long delays in obtaining their false documents, returned to Santiago instead of flying to Washington as originally planned.

Landau's cable, sent July 28 via the State Department's "Roger Channel" to bypass regular distribution routes, reached the desk of CIA Director George Bush. Bush handled the matter because Walters, who was about to retire from the agency, was on vacation in Florida. At State, the cable was routed from Kissinger's office to that of Harry Schlaudeman, assistant secretary of state for inter-American affairs.

Landau expected Walters, who had visited Paraguay on agency business only a month before, to take quick action. "I sent a message to Gen. Walters outlining the whole matter and suggesting that I presumed that this matter fell within the scope of his agency and that he was aware of all this. I said that if he did not, I suggested he deny them ['Williams' and 'Romeral'] entry at the port of arrival," Landau said in a later deposition.

The CIA reaction was peculiar. A week passed during which Townley and Fernandez, had they followed their original plan, could well have had time to arrive in Washington and kill Letelier. Walters and Bush conferred about the matter, and finally on Aug. 4 Walters called Landau in Paraguay to tell him that the CIA was "not aware" of the Chilean mission, and wanted nothing to do with it. But Walters, as far as is known, ordered no CIA action to stop the Chilean mission or control it in any way.

Judging from his actions, Ambassador Landau was alarmed. He immediately informed the State Department that the visas issued to "Juan Williams" and "Alejandro Romeral" were revoked. He demanded that the Paraguayan official who had requested the visas retrieve the passports from the Chileans and return them so that he could physically cancel the visas.

Landau considered the matter so serious that he ordered lookouts posted at all U.S. consulates and ports of entry to arrest "Williams" and "Romeral" if they tried to enter the United States and to prevent them from applying for visas in any other country. Landau also made 10 telephone calls to a high Paraguayan official over the following weeks to insist that the Paraguayans return their passports.

In Santiago, preparations began for the second and third DINA missions. Four false Chilean official passports were sent to the U.S. consulate in Santiago with government requests for visas to the United States. On Aug. 17 the visas were issued for passports in the names of "Juan Williams," "Alejandro Romeral," "Armando Faundez" and "Liliana Walker."

Although two of the names were the same as those used in Paraguay, the DINA agents using the identities of Williams and Romeral were not Townley and Fernandez. They were Capts. Rene Riveros ("Williams") and Rolando Mosqueira ("Romeral"), who arrived in Miami Aug. 22, apparently on a mission to "clear" the use of the names in Paraguay by acting as decoys to test whether U.S. authorities would react to their entering the country.

Although the two men were not stopped at Miami Airport, their arrival was detected and reported to Washington. The circumstances and records of that detection, presumably made by INS officials as a result of Landau's lookout notice, remain unclear even within the FBI.

The agents, clearly intending to call attention to their presence, informed Vernon Walters' CIA office by phone that they, "Juan Williams" and "Alejandro Romeral," were in Washington. On Sept. 1, the two men arrived back in Santiago.

What did the CIA do, if anything? We don't know. It would have been logical for those who knew of Chile's ongoing covert operation in Washington to try to find out what Chile was up to, especially in light of the "Romeral-Williams" team's claim in Paraguay that their mission had CIA clearance. It is beyond belief that the CIA would simply have ignored a clandestine operation by a foreign intelligence service in Washington, or anywhere in the United States."

DINA and the CIA were in constant touch with each other through normal liaison channels. Walters' duties as deputy director included liaison with foreign intelligence services and he knew DINA chief Contreras personally. Did he or director Bush order their representative in Chile to tell his liaison counterpart in Chilean intelligence, "Hey, we know you're up to something in Washington, so either tell us what it is or stop it"?

Moreover, it was well known in intelligence circles that DINA had carried out assassination of exile leaders in foreign countries. Given DINA's macabre reputation and Letelier's prominence in Washington, it would not be difficult to speculate that if DINA were planning an assassination in Washington, the target would be Letelier.

One thing is clear: DINA chief Contreras would almost certainly have canceled the reamaining operations to kill Letelier if the CIA or State Department had raised alarums about the "Romeral" and "Williams" missions and expressed their displeasure to the Chilean government. An intelligence officer familiar with the case said that any warning would have been sufficient to cause the assassination to be scuttled.

It wasn't. On Aug. 26, Lt. Fernandez, traveling as "Armando Faundez," arrived in Washington with DINA agent "Liliana Walker" (whose real identity has never been established). They conducted "preoperational" surveillance on Letelier.

On Sept. 9, Townley, traveling on an official Chilean passport in the name "Hans Peterson Silva," arrived to head the fourth and final stage of the assassination operation. He received Fernandez' surveillance report, then arranged with four members of a Cuban exile group in Union City, N.J., to help him build and plant the bomb. On Sept. 21 it exploded under Letelier's legs, killing him and Ronni Moffitt, who happened to be riding to work that day with Letelier and her husband Michael, who survived.

Immediately, the assassination was put in the context of prior attacks on prominent Chilean exiles opposing the Pinochet government. Two years earlier in a hauntingly similar car bombing in Buenos Aires, Argentina, the former chief of the Chilean armed forces, Gen. Carlos Prats, and his wife were murdered. One year before, exiled Christian Democratic leader Bernardo Leighton, an advocate of a leftist-centrist front against Pinochet, was shot down with his wife on a Rome street. Both survived. DINA was widely believed to have been responsible.

It would seem to go without saying that those who had detected DINA's covert operation in Washington prior to Letelier's assassination would immediately tell the FBI all they knew. The passport photos of "Romeral" and "Williams," the Paraguay incident and the actual entry into the United States of Chilean intelligence agents were obviously important leads worthy of highest priority in the investigation.

Moreover, the FBI's man in South America, Special Agent Robert Scherrer, made a major intelligence breakthrough one week after the assassination. He reported that Chile had organized a six-nation intelligence network called Operation Condor, whose functions included interchanging passports for use on missions to assassinate exiled leftist leaders. Paraguay was one of the members, with Argentina, Uruguay, Brazil, Bolivia and Chile. Scherrer, even without knowing about the "Romeral" and "Williams" affair in Paraguay, concluded in his Sept. 28 cable to Washington that the Letelier assassination "may have been carried out as a . . . phase of Operation Condor." Scherer's cable was distrubuted to the CIA and State Department.

What then did Bush, Walters, Landau and others in State and the CIA do with the "Romeral" and "Williams" photographs and information after the assassination?

The Letelier investigation had been assigned to Assistant U.S. Attorney Eugene Propper and Special Agent L. Carter Cornick of the FBI's Washington field office. Propper, realizing that the investigation could not go far without cooperation from the CIA, met Bush two weeks after the assassination. According to one of those present in that meeting, Bush talked about the importance of Operation Condor to the Letelier case, but did not say a word about the "Romeral" and "Williams" pictures and the Paraguay incident. Nor did Bush, Walters or anyone else from the CIA subsequently volunteer their information about Chile's covert missions to Washington.

Instead, the CIA seems to have done just the opposite. Stories appeared in Newsweek, The Washington Post, The Washington Star and The New York Times saying the CIA had concluded that DINA had nothing to do with the Letelier assassination. In one of the stories, Bush was reported to have personally informed Kissinger of his conclusions about DINA's innocence.

The sources cited in the articles suggested a "martyr theory" for the assassination, according to which leftists may have killed Letelier in order to create a martyr and discredit the Chilean government at a time Pinochet was improving his human rights image.

At the State Department, some but not all of the pertinent information about Chile's secret missions was turned over to the FBI on Oct. 22, one month after the assassination. The information included copies of the photographs of "Romeral" and "Williams" and the fact that two men using those names and official Chilean passports had entered Miami on Aug. 22 (but not that they had also come to Washington).

But the "Romeral-Williams" information and photographs played no active role for the first 10 months of the FBI investigation. When, in July 1977, the photographs were finally put to use, the "Williams" picture was identified and the case was on the way to being solved. The man in the picture, Michael Townley, was turned over to the FBI the following March in accord with a secret agreement signed by Chile under heavy U.S. diplomatic pressure. Townley confessed and became the presecution's chief witness.

In the course of the investigation, there were at least five cases of withholding, destruction or concealment of key evidentiary documents. These incidents raise the possibility that an attempt was made from within the U.S. government to sabotage the FBI investigation and divert its focus away from Chile's military government:

1. For more than a year after the assassination, Assistant U.S. Attorney Propper and the FBI did not receive Ambassador Landau's cable to Vernon Walters fully explaining the Paraguay incident.

2. State Department Chile desk officer Robert Driscoll, who told a superior in a memo that "Romeral" and "Williams" were in Washington around the time of the assassination, ignored instructions to inform the FBI. The memo was given to the FBI from Chile desk files more than a year after the assassination.

3. Immigration and Naturalization Service information -- based on I-94 forms filled out by all foreigners entering the United States -- on three of the five members of DINA's assassination missions were removed from INS computers. The missing listings were "Romeral," "Williams" (the Aug. 22 Miami entry with Chilean passports) and "Hans Petersen" (the name used by Townley to enter New York Sept. 9, 1976). Moreover, INS officials conducted a file search in 1979 and discovered the disappearance of all paperwork that normally would accompany lookout notices such as those the State Department ordered posted for "Romeral" and "Williams."

4. Someone with access to U.S. citizen registration files in the U.S. consulate in Santiago removed the photograph of Townley on file there. u

5. Other evidence in the consulate files was destroyed as well. After Townley's expulsion, FBI agent Scherrer discovered that U.S. Consul Josiah Brownell had ordered the shredding of consular files that would have included the letters from the Chilean Foreign Ministry requesting visas for agents "Hans Petersen," "Armando Faundez" and "Liliana Walker." In mid-1977, Scherrer had warned Brownell that the files might contain evidence in the Letelier case and should not be destroyed.

The unanswered questions do not diminish the achievement of the U.S. investigators who solved the assassination and whose evidence stood the test of a jury trial in which three Cuban exile accomplices were convicted. But the actions taken willfully to divert the investigation from its course and delay it for at least a year are also crimes. Those actions should be subject to the same scrutiny as the assassination itself. If there are innocent explanations, they should be made public along with all relevant documentation in the case. Only then will the prosecution of the Letelier-Moffitt murders stand as untainted examples of the triumph of American justice.