First-hand accounts of events surrounding the slaying last December of four American missionaries in El Salvador suggest that the killings may have been ordered and carried out in a planned operation by El Salvador's U.S.-backed military forces.

One report is of an intercepted radio transmission between two units of Salvadoran security forces on a stake-out at the San Salvador airport, where they were apparently awaiting an unidentified woman. That woman is believed by those who reported the conversation to have been one of the murdered missionaries.

The heretofore unpublished information, based on interviews with Salvadoran and American officials and church workers who were in El Salvador at the time of the killings, contrasts sharply to the theory of the crime provided by U.S. government officials. That theory holds that the killings were spontaneous acts of violence and that Salvadoran military personnel involved, if any, were enlisted men acting without higher orders.

Maryknoll sisters Ita Ford and Maura Clarke, Ursuline sister Dorothy Kazel and lay missionary Jean Donovan disappeared Dec. 2, 1980, after last being seen at El Salvador's international airport, 40 miles from the capital, San Salvador. Their burnt-out Toyota van was found along the airport road the next day, and the four women's bodies were exhumed from an unmarked common grave in a remote area on Dec. 4.

The U.S. government pressed publicly for a full investigation of possible security force involvement. Secretary of State Alexander M. Haig Jr., at a March 24 congressional hearing, explained the deaths as accidental overreaction by nervous soldiers who "may have misread the mere travelling down the road [of the women's van] as an effort to run a roadblock."

In April, after a six-month investigation, six security force enlisted men were placed under provisional arrest in El Salvador on suspicion of involvement in the crime.

People with direct knowledge of the events surrounding the women's killings, located and interviewed in the United States, provided information pointing to foreknowledge and planning of the killings by the Salvadoran military.

Robert E. White, former U.S. ambassador to El Salvador, who handled the case for the United States before being fired from his post by the Reagan administration in January, revealed that several weeks after the killings a "high-ranking Christian Democrat in the Salvadoran government" told him the contents of a radio transmission intercepted the day the women were killed.

The transmission which White said was monitored by military personnel opposed to the rampant violence by the security forces, was a conversation between two security force units in the area of the international airport several hours before Maryknoll sisters Ford and Clark arrived on a flight from Managua, Nicaragua. It contained the words, "No, she didn't arrive on that flight. We'll have to wait for the next."

White declined to give further details, but said it is a "legitimate inference" that the message referred to Ita Ford, who, he reasoned, may have been singled out for execution because of her work distributing food and medicine and protecting refugees in radicalized Chalatenango province, north of San Salvador.

White said the intercepted message is only one report, but said it is important as a lead because, if more than one military unit were involved in capturing and executing the women, it would indicate a military operation under the control of at least one superior officer.

He said he reported all he learned about the case in cables to the State Department, and that it is up to the department to release information or use it in the investigation.

The radio message appears to complement other accounts of events surrounding the killings:

The day of their deaths, Donovan and Kazel drove to the airport to pick up Ford and Clarke, who were returning from a Maryknoll regional meeting in Managua. The two returning nuns, however, had missed the first Managua-El Salavador flight that day. Donovan and Kazel picked up two other arriving Maryknollers, but had to return to the airport to meet Ford and Clarke, who arrived on a flight several hours later.

Still another Maryknoll nun, Sister Marie Rieckelman, a U.S.-based psychiatrist who had attended the Managua meeting, also was on the first flight, and was continuing on it to Miami.

In a recent interview in Washington, she said she and nine other in-transit passengers were made to remain on board during the 40-minute layover, and that during the wait three uniformed Salvadoran soldiers boarded the plane, a highly unusual occurrence even in Latin American countries ruled by military governments.

She said she was the only American woman on board, and that the soldiers scrutinized her closely and instructed the stewardess to question her twice about where she was going.

Also that day, 70 miles north of San Salvador, in the city of Chalatenango, parish priest Efrain Lopez, with whom Ford and Clarke had been doing refugee relief work, said he received two death threats to himself and the nuns so severe that he asked his superiors to relieve him of the post.

Now an exile in the United States, Lopez said in an interview that one of the threats was made the evening of Dec. 2 to a parish worker by an unidentified man who showed a list with the names of Lopez, Ford, Clarke and other church workers, and said, "Here is a list of the people we are going to kill. And today, this very night, we will begin."

Carlos Paredes, then the Salvadoran deputy minister of planning, described a meeting in the Blue Room of the presidential palace two weeks before the murders in which he listened to a half-hour presentation by Defense Minister Jose Guillermo Garcia intened to prove to civilian officials that the nuns and priests in Chalatenango were collaborating with the leftist guerrillas.

Although such charges have been made repeatedly by the Salvadoran military, no substantiation has been produced. The nuns' activity in Chalatenanto has been strongly defended by San Salvador Archbishop Arturo Rivera y Damas.

The April arrests of the six Salvadoran security force men are still somewhat of a mystery, according to the FBI, which El Salvador brought into the case at the urging of the United States.

An FBI official assigned to the case, who asked not to be identified, said the FBI role was confined to technical help in fingerprint and ballistics analysis and that the arrested men's fingerprints and rifles were not submitted for FBI comparison tests until after the arrests.

Only then was it established that one of the men's fingerprints matched prints found on the burned bus, and that another man's rifle produced ballistics data matching cartridge cases found near the burial site.

A Salvadoran judge is examining evidence to determine if formal charges can be brought against the six men.