Three days before a bomb blast killed 241 American servicemen at the Beirut airport Oct. 23, a Lebanese financial emissary named Hassan Hamiz was given a check or voucher for approximately $50,000 that could be cashed only at the Iranian embassies in Beirut or Damascus, according to reliable intelligence reports. The reports indicate that, after the bombing, it was cashed at the embassy in Damascus, where Hamiz, described as a "fixer" with high- level contacts, had very close relations with Iranian Ambassador Ali Akbar Mohtashami, who has been identified by the CIA as a key figure in Middle East terrorism.

The $50,000 payment is believed to be a primary link in the financing that set in motion two fanatical suicide truck-bomb attacks that destroyed the U.S. Marine emplacement and also killed 58 soldiers at the French military headquarters in Beirut that same October morning, according to a review of intelligence documents and interviews with officials in the United States, Middle East and Europe.

Investigations by the CIA and French, Israeli and Lebanese intelligence services have pieced together many of the essential details of the devastation of Oct. 23, perpetrated by men who crave death as soldiers of their God and planned by others who rely on the terror factor as the most effective brand of political warfare. The events that ended with the blood and rub-ble of that massive explosion included a complex series of transactions, codenames, meetings in Beirut, the Bekaa Valley in Lebanon and Damascus, and trucks moving explosives under cover as rice transports.

In addition to Hamiz, 13 individuals now have been tied to the bombings by the intelligence services. They include a Syrian intelligence colonel, a former PLO security officer, Syrian members of the Syrian-controlled Saiqa (Thunderbolt) Palestinian terrorist organization, a relative of the Shiite Moslem leader in Lebanon's Bekaa Valley, an Islamic fundamentalist clergyman from Beirut and several veterans of other terrorist operations. Among them are:

* The Syrian intelligence officer, identified by various intelligence organizations as Lt. Col. Diyab (also spelled Diab), has been traced to a planning meeting Oct. 21 or 22, just before the Marine bombing. Surveillance reports show that he was in the southern suburbs of Beirut and was planning an attack against French and American installations.

* A key architect of the operation, according to Israeli intelligence, is identified as Nablan Shaykh, a former deputy chief of national security for the Palestine Liberation Organization. He operated under the code name Abu Kifah and had been in charge of security in a west Beirut neighborhood at the Museum Crossing on the line dividing Christian eastern and Moslem western Beirut.

* Two other Syrian officials in Saiqa, a PLO organization founded and controlled by the Syrian military, attended meetings on Oct. 21 and 22 and discussed a strike against the multinational forces in Beirut.

One Saiqa member, Ahmed Halaq, is identified in intelligence reports as a specialist in assassinations. He had been in Lebanon's Bekaa Valley, a Syrian-controlled area about 30 miles east of Beirut that is a known staging ground for terrorists. Halaq was placed in the Bekaa region about a week before the Oct. 23 bombing.

The second man is Bilal Hasan. Within days of the suicide attacks, these two men were later found to have visited the Palestinian refugee camps of Sabra and Shatilla, the former PLO neighborhoods where hundreds of people were massacred by Phalangist militia in 1982. The reason for their visit has not been determined, but Israeli intelligence sources claim that weapons and explosives were still stockpiled in such camps.

Halaq and Hasan were tied to attempted terrorism in Egypt in 1979, when two other ter- rorists were arrested in Cairo with toothpaste tubes filled with high explosives. After confessing, the two terrorists identified Hasan as the person who recruited them and Halaq as a Syrian intelligence captain who prepared their explosives.

* Abu Haydar Musawi, a cousin of a Shiite leader in the Syrian-controlled Bekaa Valley, visited Beirut several days before Oct. 23 and was involved in obtaining the pickup trucks used in the bombing. According to intelligence reports, he heads what is called the Hussein Suicide Commandos, and intelligence reports say that immediately before or right after Oct. 23 he claimed he was going to report the outcome of the planned operations to his cousin Hussein Musawi, in the Bekaa Valley. Hamiz, the Lebanese financial emissary who cashed the $50,000 check after the bombing, is also close to Hussein Musawi, the reports indicate.

Hussein Musawi has previously been named in press accounts as one of the people involved in the bombing. He has denied a direct or indirect role but praised the bombing by saying: "I salute this good act and I consider it a good deed and a legitimate right, and I bow to the spirits of the martyrs who carried out this operation."

* Mohammed Hussein Fadlallah, the second person named in press accounts, is the leader of the Hezballah (Party of God), a militant Shiite movement based in Beirut's southern suburbs. The movement embraces remnants of the radical Al Dawa (The Call) party, which is now based in Iran.

Fadlallah has close ties to the government of Iranian leader Ayatollah Ruhollah Khomeini. He had frequent dealings with the Iranian Embassy in Beirut until the Iranians were expelled following the Oct. 23 bombings. At the time of this expulsion, Fadlallah led mass demonstrations in Beirut protesting the action by the Lebanese government. Hussein Musawi is considered to be Fadlallah's lieutenant and principal military commander.

Fadlallah and the key planner, Nablan Shaykh, attended a planning meeting Oct. 20 at the Soviet-Palestinian friendship house in Damascus, which since last summer has been used by dissident PLO leaders. They discussed attacks against the multinational force in Beirut, according to intelligence reports.

One week after the bombings, in an interview with Washington Post correspondent Herbert H. Denton, Fadlallah denied any involvement. He said any such charges were a frame-up by Christian Phalangist militiamen and the Lebanese Army intelligence. During the interview in Beirut's Shiite Moslem slums Fadlallah was guarded by men holding AK47 Kalashnikov assault rifles. Asked about the show of force by a man who said he was interested only in peace, Fadlallah laughed. "This," he replied, "is for people who don't understand my concept of peace."

Two weeks ago Fadlallah could not be located in Beirut, and sources said he was in Tehran for meetings with government officials. The speaker of the Iranian parliament, Hojatoleslam Ali Akbar Hashemi Rafsanjani, met with Fadlallah yesterday, according to Reuter, and urged the recruitment of more "self-sacrificing youths" to carry out suicide attacks on the U.S. and French peace-keeping forces in Lebanon. According to Reuter, Fadlallah replied: "Our aim is to expel the aggressive forces of the United States and other so-called multinational forces including the Zionists."

Intelligence reports indicate that on the night of Oct. 22, just hours before the bombings, Fad- lallah received--either in his apartment or at his mosque in Beirut--the two men who drove the trucks on the suicide bomings. He blessed them, gave them so-called deeds or guarantees to their places in paradise, where they would have eternal joy. According to one resident in Bir Abed, the suburb in southern Beirut where Fadlallah lives and has his mosque, as recently as two weeks ago Fadlallah preached to 300 fighters, aged 14 to 30, and promised any man who killed an Israeli in southern Lebanon a place in paradise.

* Ahmed Qudura, a former PLO Fatah member, and a second man identified in intelligence reports only as Umayah, a security officer of the rebel PLO group led by Abu Musa, have also been tied to the planning sessions on Oct. 21 or 22 attended by chief planner Shaykh and Diyab.

The intelligence agencies have established that four others collaborated in the bombings. One of them obtained the explosives after being turned down by a Lebanese supplier. All four were followers of Hussein Musawi whose loyalty to him had resulted in their expulsion from the mainline Shiite group in Lebanon called Amal. Hussein Musawi formed a rival organization called Islamic Amal.

Two of the four, Ali Fatuni and Ibrahim Aqil, had been accused in the July 1983 terrorist attack on Lebanese Prime Minister Shafiq Wazzan, in which a remote-controlled car bomb only partially detonated and no injuries resulted. The five suspects arrested in the assassination attempt named Aqil as the man who rigged the car with 154 pounds of explosives. He has also been tied to the Nov. 4, 1983, suicide-bombing of the Israeli Defense Forces interrogation center at Tyre, in southern Lebanon, in which 29 Israelis and 32 Arab detainees were killed.

The other two are Ali Majid and Wafiq Safa. It was Safa, intelligence reports say, who approached Lebanese and Palestinian suppliers several days before Oct. 23 to request about 4,000 pounds of explosives. He specified that the explosives were for a special operation and that they had to fit into two pickup trucks. According to a Middle East intelligence officer, Safa was asked at this meeting whether the special operation had been approved by the Islamic Amal leadership. Safa replied that, as a follower of Hussein Musawi, he was "immune" to Islamic Amal, and that the operation was authorized by Syria and was being organized as a result of Syrian "inspiration."

In all, Safa claimed he had three trucks with forged documents and falsified markings from TMA, Trans-Mediterranean Airways, a Lebanese cargo airlines. The cover was perfect, he said, because the trucks were precisely the type normally used to transport rice and other goods. These trucks were known by all and would not be stopped, the reports state he said. In the actual bombings, a Mercedes Benz truck was used at the Marine headquarters and a red van was used at the French headquarters.

The Lebanese declined to provide the explosives. An apparent effort to smuggle in the explosives from the Bekaa Valley was also unsuccessful. According to Israeli intelligence, Safa and his men tapped into secret caches still in west Beirut kept by persons loyal to PLO rebel leader Abu Musa, whose full name is Col. Said Musa.

The Israelis, after invading Lebanon in June 1982, had little time that September to attempt to clean out such caches in Beirut, once the agreement was reached that resulted in their withdrawal from Beirut. It is widely agreed that such arms and explosive stores still exist in the Lebanese capital and surroundings. For example, the Pentagon's investigation into the Marine bombing, released in December, said: "Stockpiles of explosives, built up over a decade prior to the Israeli invasion of June 1982, are reportedly still in place and available for future terrorist operations in and around Beirut."

According to the intelligence reports, these stockpiles provided the explosives that killed the marines.

The FBI, which was called in to assist in investigating the Marine bombing, and other intelligence agencies have determined that the explosives, which had the equivalent of 12,000 pounds of TNT, included the powerful plastic, PETN, tied into a sophisticated gas-enhancing construction that employed propane gas bottles to greatly magnify the blast. Another plastic explosive, hexogen, was used in the French bombing. The use of PETN and hexogen, highly restricted materials that are hard to obtain outside military channels, strongly suggests the involvement of government and military intelligence services.

A top-secret intelligence source reported just before Oct. 23 that Lt. Col Diyab was plan- ning a terrorist act against unknown installations of the French and American forces on Oct. 23. The substance of this report--including even the date of a suspected attack, according to one account--was passed to the forces. But it was one of nearly 100 such warnings the multinational forces had received and it did not specify the target. Multinational officers, who had dealt for months with such threats that had not materialized, concluded little could be done.

The Pentagon commission investigating the bombing concluded that the Marine commander "was not provided with the timely intelligence, tailored to his specific operation needs, that was necessary to defend against the broad spectrum of threats he faced." In addition the commission said that the United States did not have control over sufficient human intelligence--agents, informers, traditional spies--to track down warnings and obtain information on targets and methods.

Haydar Musawi maintains a business office in west Beirut on Assad Street. There, intelligence reports show, explosives were loaded or unloaded into three pickup trucks, one believed to be the yellow Mercedes Benz stakebed truck used in the Marine bombing.

In addition to the Marines and the French, the suicide commandos initially had two other targets. One was the Lebanese parliament; the other has not been determined.

Those operations did not take place, and the intelligence reports do not indicate why. The reports show that after Haydar Musawi said he was going to report on the operations to his cousin, Hussein Musawi, he claimed he would return to "explode the situation in Beirut."

Intelligence officials are perplexed by a reliable intelligence report that Haydar Musawi asserted that he lost three of his members in the Oct. 23 operations. Lone drivers were involved in the Marine and French bombings. It is not known how or why a third commando might have been lost. One analyst said a third may have been killed in a practice run.

Intelligence officials also place some significance on the intercepted communications, which show Haydar Musawi emphasizing that the attack on the French and U.S. forces was not so much to remove the forces from Lebanon, as is widely believed to be the motive, but rather because France was shipping arms to Iraq and supporting Iraq in its war with Iran. According to the reports on Haydar Musawi's assertions, the U.S. forces were attacked because the Americans were not opposing the French decision to back Iraq.

Israeli officials have called atten- tion to this apparent motivation. Other officials said it was cer- tainly one of the reasons, but by no means the exclusive or even chief reason for the bombing. Citing widespread Syrian complicity in the Marine bombing, the officials said that the bombing served multiple purposes, and the Syrian policy is clearly to get the United States and entire multinational force out of Lebanon.

The intelligence agencies, using communications intercepts and other highly classified methods, have determined that the strongest Syrian connection to the bombing is Lt. Col. Diyab. He used a code name, Abu Nidal, which is also the name of a well-known international terrorist who has no known connection with the Marine bombing. Officials believe the code name was cleverly used to provide a false trail. To further confuse the picture, it turns out there are two Syrian intelligence officers with the last name Diyab, and the intelligence agencies are not sure which one was involved in the bombings.

One of the two is Lt. Col Abd Gabar Diyab, who served in the Syrian Embassy in Paris under an intelligence cover as a second secretary in 1980.

The second is Muhammed Khayr Diyab. In April 1982, he replaced the Syrian military attache in Paris who had been expelled by the French after a car bomb exploded in Paris killing one person and injuring 63.

Much else is still not known and may never be known about the bombing and those responsible. Since the intelligence gathering was done after the Oct. 23 bombing, and because much of the material is based on highly classified intercepts of communications, sources and agents, officials said it is unlikely that any official or public case would be made by the United States or any other government for some time.

If a case had to be made in a court of law, several officials in U.S. and foreign intelligence agencies said, they might not be able to convince a jury. Several others said that the wealth of detail and the volume of circumstantial evidence could make a provable case. This is probably what led Secretary of Defense Caspar W. Weinberger to say publicly a month after the bombing that those responsible were "basically Iranians with sponsorship and knowledge and authority of the Syrian government."

That was the strongest public statement made by any Reagan administration official connecting the bombing to Iranians and Syrians. Officials have said that Weinberger opposed the Marine presence in Lebanon and, as the defense secretary, felt a deep personal responsibility for the death of 241 men under his command.

Other Reagan administration officials, particularly those in the White House and State Department who have to deal diplomatically with Syria, have kept their distance from such a direct, public accusation. Diplomatic officials note that Syria will be central to any negotiated settlement that might result in a face- saving withdrawal of the U.S. Marines from Lebanon.

After the bombings, a large number of people were observed rushing out of the Iranian Embassy in Beirut into waiting cars that sped off to an unknown destination, presumably the southern suburbs controlled by Fadlallah.

"It only takes them three minutes to get out of government-controlled areas," said a Lebanese intelligence source.

Soon after the incident, an unknown group calling itself the Islamic Holy War claimed responsibility in telephone calls to news agencies. Intelligence officials said they are virtually certain no such operational group exists, but that it is a psychological warfare arm covering the terrorist acts of various Islamic groups. Two days after the bombings, the group published a statement in Beirut newspapers that intelligence officials said quite accurately characterizes the degree of fanaticism felt by some--but only some--of those involved in the bombings. The published statement said:

"We are the soldiers of God and we crave death. Violence will remain our only path if they the mulitinational forces do not leave. We are ready to turn Lebanon into another Vietnam. We are not Iranians or Syrians, or Palestinians. We are Lebanese Moslems who follow the dicta of the Koran."