Three years after the Bay of Pigs, here is the first fully documented story of a key event in that disaster:

While Castro was making his revolution, Andrew St. George spent more time with him in the mountains than any other American journalist. They were close friends - Castro once awarded him a medal, ordered him released when he was arrested. Since Castro's open avowal of Communism, St. George has maintained close contact with anti-Castro groups.

This week will mark the anniversary of the ill-fated disaster at the Bay of Pigs. It is exactly three years since Fidel Castro's regime threw back an exile-manned, U.S.-supported attempt to invade Cuba.

The story of that debacle has been repeatedly discussed since. It has been the subject of Congressional and executive investigations and of partisan political recrimination.

Yet one of the most important details of that Cuban defeat has not previously been revealed. It is an event that may have been the whole key to the Bay of Pigs tragedy, and its occurrence-or failure to occur-had a profound effect on the invasion itself and on subsequent history. And although it has not publicly been acknowledged, long and painstaking investigation by this reporter has documented this event.

Carried out on the highest levels of Cuba's revolutionary government, it was an attempt to assassinate Fidel Castro. And it came within a cat's whisker of success.

This plot, of course, was not the first against Castro's life, nor has it been the last. One of the records of which the bearded revolutionary leader is least proud is the number of times he has been the target of nearly successful assassination attempts.

Before detailing the most important plot, let's look at a few others. The most recent try came just before the celebrations in Havana last January commemorating the victory over Batista. U.S. security boats intercepted two speedboats crammed with anti-Castro conspirators and hundreds of petacas, plastic bombs to blast Castro from his reviewing stand.

The U.S. government, worried about the Caribbean aftermath of a successful assassination, is not happy about such attempts. But American nervousness has not been able to do too much about it. Some of the attempts have come so close to success that Castro has been left with the apprehensive wariness of a lone fox in a hunting preserve.

An early try at an ambush was engineered by the sinister Col. Johnny Abbes, formerly intelligence chief of the Dominican Republic. Abbes, working on orders of Dominican strongman Rafael Trujillo-himself the victim of assassination-hired a swashbuckling American adventurer, Alex Rorke, son-in-law of New York's famed restauranteur, Sherman Billingsley, to pilot a speedboat that landed eight men before dawn in eastern Cuba. The plan was to ambush Castro on his way to speak at a service at the Santiago cemetery.

Through a pouring rain, Trujillo's Tommy gun team spotted Castro's chief bodyguard, Capt. Alfredo Gamonal, in the second jeep of a caravan.

The killers assumed Castro was in the back seat, and their bullets chewed up Gamonal, the superintendent of cemeteries and the jeep driver. Castro, riding in the next-to-the-last jeep, was unhurt.

"He may have nine lives," Abess told Rorke, who returned to Ciudad Trujillo complaining of Castro's charmed life. "But if so, I'll try a tenth time."

Abbes acquired an apartment in Havana overlooking the CMQ television studios, where Castro appeared frequently to deliver his nation-wide harangues. Another American adventurer, a one-time top competition sharpshooter, was retained by Trujillo on a down payment of $25,000 and the promise of a cool million if he managed to score a clean hit on his moving target.

The marksman said he could do it, but demanded a special weapon-a bench-adjusted telescopic carbine with a nondeflecting muzzle silencer.

"Dominican ordnance experts immediately went to work to produce the rifle," former Dominican State Security Minister General Arturo Espaillat recalls. "The weapon was completed and en route to Cuba when Trujillo canceled the project . . . He was afraid of Washington's fury. I really think that Fidel would be dead today if the plot had not been called off."

Prior to that attempt, another American, Alan Robert Nye, a 31-year-old Chicagoan, was convicted in Havana for conspiring to kill Castro. Fee: $100,000. Although a Cuban court had signed, sealed and delivered the order for his execution, Nye was allowed to leave the country for the U.S.

There have been far too many of these attempts to detail here; although men like Alex Rorke, and Paul Hughes, a former American Navy jet pilot, have lost their lives because of them, Castro cannot rest easy.

Before embarking on an airplane trip, he usually inspects the plane from tip to tail. During the warm-up, he once spotted flames belching from the engine exhaust. Castro ordered the ignition cut and both pilots back into the cabin, where they explained for a half-hour that burning exhaust was normal and that it did not prove the plane booby-trapped.

During his visit to New York to attend the United Nations in 1960, Castro's food problems were magnified by his methods of selecting restaurants. A brace of bodyguards was ordered to go out and buy food from a restaurant-but never from the hotel kitchen or from the restaurant nearest the hotel. On each occasion, Castro would call out a number, say, "Three!" or "Five!" which meant they had to count off three or five restaurants before they could enter the next one, thus having presumably eluded the potential poisoners.

His security chief also carried sensitive white mice-"to detect assassination attempts by radiation or nerve gas," chief body guard Gamonal explained.

But the only security measure Castro really has faith in is the one he learned in his two years of guerrilla warfare: never let anyone know where you'll show up next. In the Sierra Maestra, when Castro and his little band were making their revolution against Batista, no one but Fidel knew exactly where the day's march route would end.

The habit persists. When he made his first visit to Moscow, he left Havana and returned to it as secretly as an enemy infiltrator. No one in Cuba knew when to expect the Premier home. When his Russian airliner finally landed, there was nobody to welcome him except some startled airplane mechanics. Grinning, Castro borrowed a coin, dropped it into the nearest pay phone to let Cuban President Osvaldo Dorticos know he was back.

But is was the assassination attempt just before the Bay of Pigs that was the most significant of all. It involved several senior commanders of the Cuban Revolutionary Armed Forces as well as key civilian leaders.

The Central Intelligence Agency, which had received absolutely reliable reports that a conspiracy to assassinate Castro was developing among his top lieutenants, decided to contact the plotters, because the U.S. was already training its own anti-Castro force in Guatemala. CIA agents discovered the conspiracy had a wealthy contact man in Miami, a former sugar cane grower, Alberto Fernandez.

With CIA's tacit approval, Fernandez bought a converted subchaser, the Texana III, and had it outfitted with concealed deck armaments, 50-cal. machine guns, two 57 mm. recoilless rifles and a pair of small speedboats with muffled interceptor engines.

Now began one of the most daring and extraordinary secret intelligence operations ever attempted. Shuttling in the dark of night between Marathon Key and the north coast of Cuba, the Texana III was the link between the Cuban conspirators and the U.S.

Its two deck boats skimmered up to shore less than a dozen miles from Havana to pick up their unusual passengers: Cuban rebel comandantes in full uniform and governmental functionaries carrying brief cases.

Before the sun came up, the travelers were in U.S. waters, where they held quick conferences with American agents, sped back to Cuba the next night.

The tricky and hazardous process went on for a couple of months, and the U.S. learned more and more about the murder conspiracy beaded by cool, brainy Comandante Humberto Sori Marin, a hero of the Castro revolution. Other top-level men involved astounded the Americans: Secret Police Chief Aldo Vera; Comandante Julio Rodriguez, deputy commandant of the San Antonio de los Banos air base; several Navy flag officers; the military superintendent of Camagüey Province; the president of the Cuban Sugar Institute; and the undersecretary of finance. They were determined to act early in 1961. The plot was to kill both Castros and touch off a general uprising.

Convinced that, regardless of what the U.S. did, the conspirators meant business, the CIA decided to capitalize on the plot without actually participating in it. Officials readied the landing forces to go ashore at the same time. Agents began a series of secret meetings in Havana with the conspirators to coordinate their plans.

Then, just before the target date, there occurred one of those impossible mistakes nobody ever believes. A crucially important secret conference was being held with most of the top conspirators. They met in a house of known safety in Havana's Miramar suburb on a tranquil street, Calle Once. It was a large, yellow, somnolent building, lived in and owned by a respectable retired sugar engineer and his wife.

In the front patio, the engineer played gin rummy with his wife and led by many points. In the back of the house, the plotters gathered around a heavy refectory table covered with street maps, pinpointing the massive incendiary attack against the crowded downtown district of "Old Havana," which was to touch off the uprising. The Texana III had already shipped in hundreds of petacas.

Several blocks away, a militia security patrol stopped in front of another house, then entered to search it. A nervous woman in a back room fled from a rear door with her small daughter. She ran beneath garden walls and ducked into the rear entrance of the large yellow house of the engineer, an old friend.

The street was deserted. But one militiaman watched as she ran to the yellow house. So, under the blazing sky of a spring afternoon, in Miramar, the security unit walked down the street to that yellow house, that sleepy yellow house . . . .

The pity of it was that the nervous woman who ran did not have to. The security police were on a routine search. She was suspected of nothing; if she had remained, nothing would have gone wrong.

The 11 key figures of the Sori Marin conspiracy were caught in a single sweep. The four men who had been sent in by the CIA might have gotten away; they were all Cubans and carried such perfectly forged papers that two were subsequently shot under their assumed names.

But Sori Marin had no chance whatever. As the milicianos burst into the room, his pistol leaped into his hand. But the security men's snub-nosed Czech Tommy guns chattered and Sori Martin crumpled as he tried to crash though a window.

And it was all a mistake. The militia walked in by mistake. The woman ran away by mistake.

Washington working with fragmented information, decided it was too late to halt the invasion troops staging for departure in Guatemala. There was no way to know just how badly the conspiracy had been crippled; there was a possibility that many of its members had not been identified and would thus be able to carry out the plans.

It was a forlorn hope. April 17, at dawn, the first of the invasion troops splashed through the surf onto Giron Beach. April 17, at dawn, the seven top conspirators, led by Sori Martin, wounded and supported by his guards, but still wearing his uniform, were executed in Havana. Within the next few hours they were followed to the wall by the captured CIA men. The rest, the slaughter at the Bay of Pigs, is history.

U.S. security and intelligence agencies are now more worried about the possibility of a successful assassination. For Washington-which once gave tacit support to Sori Marin-now feels that a real explosion involving Castro could trigger the most unpredictable chain reaction of the coming year, a chain reaction that conceivably could turn into World War III.

The current approach was pointed up in a quiet sort of way the day Allen Dulles-whose own job as head of the CIA ended a short time after that ill starred invasion-appeared in public for the first time to talk about it on Meet the Press.

"Mr. Dulles," the moderator asked, "in launching the Bay of Pigs invasion, you were obviously expecting a popular uprising to support it. Yet none occurred. How could you have been so wrong?"

"A popular uprising?" Mr. Dulles puffed on his pipe. "That's a popular misconception-but no, I wouldn't say we expected a popular uprising. We were expecting something else to happen in Cuba... something that didn't materialize."

As this is written, U.S. intelligence is still expecting it to happen, but the expectation has now turned to a nervous and gnawing worry.